For the first time in its decade-and-a-half of production, Chevrolet's Corvette--billed as "America's only true sports car"--is being challenged for the title. Shelby American's G.T. 500 Mustang-based sports fastback has all the earmarks of a "purist" type sporty car with a just-right combination of comfort and convenience features.

Following Shelby American's announcement of a 428ci V-8-engined GT, we were skeptical if this car would be suited for street duties. We've driven many small cars with huge engine transplants, and somehow they always seemed to lack tameness during regular operation--unless regular operation was a full-throttle run down a 'strip. We found out, though, just how wrong we were after depositing ourselves behind the custom wood steering wheel of an air-conditioned G.T. 500 with Cruise-O-Matic transmission.

We could hardly believe its smoothness in bumper-to-taillight traffic and had difficulty realizing it was built by the same people who used to peddle a rough-riding, hard-steering Mustang fastback with Bunyanesque brake pedal and an engine that would outshout a John Deere tractor.

In view of this, we arranged for an even hairier G.T. 500, and also set up an equally equipped Corvette to see if there really was a challenge.

The basic Shelby G.T. 500 is a true sophisticate compared with earlier cars built by the famous Texan. There was a brief period when the Ford Cobra, also built by Shelby, was thought to be a fair and equal competitor to the Vette, but a too-high price and rather impractical design for a street-destined car put it out of contention. Chevy has cause for some worry from the G.T. 500, though. There's more room inside, and it's easier to drive in traffic. There's a civilized luggage compartment that's accessible from the outside, and there's at least the same measure of racy styling. The G.T. 500 runs quieter, and in general is a more practical car for everyday use.

The Corvette can't be sold short, though. Even in stock condition, it outruns the G.T. 500 by a solid 0.7-second in the quarter-mile, stops in less distance from any speed, is easier on gas, and exhibits noticeably better workmanship throughout.

The Vette started life back in 1953--before the world or Ford Motor Co. had ever heard of a car called the Mustang--and was then equipped with an impotent six-cylinder engine and fiberglass body. The 'glass body has remained, but the sixes were dropped two years later and the car since then has steadily climbed higher and higher in the esteem of performance addicts. Equipping the Corvette with a 427ci engine was a natural move for Chevrolet, and for the past two years this version has been the image-maker of the line. Previous editions of the 'glass-bodied car were offered with as much luxury built in as speed and performance, and we remember a statement concerning Corvette sales cutting into Cadillac. The luxury is still there, but a hard ride comes with it, so we doubt if Cadillac is still worried.

There's more potential for making the Corvette a hard runner than there is the G.T. 500. The 428ci V-8 in the Shelby car is too heavy for serious work, and it has a rather restricted breathing system compared with the 427ci Chevy V-8. Called "porcupine" by Chevy engineers, the valve layout of the Vette engine is more ideally suited to getting big charges of air/fuel mixes into the combustion chamber, and the head configuration above the chamber is a "semi Hemi" type, similar to early Chryslers.

Weight distribution is better on the G.T. 500, with 56.4 percent being carried on the front wheels. The two total out almost equally, but so much of the Corvette is the front, that it is distinctly nose-heavy. This characteristic is abruptly apparent when one attempts to leave the line under hard throttle, or when you bring it around a sharp bend while applying power. The rear end becomes frighteningly "light."

Starting with two basic cars, more is standard equipment on the G.T. 500 than on the Vette, but there's some chicanery involved. For the base price of $4,195, a G.T. 500 buyer gets the 428 engine, front-disc/rear-drum brakes, full instrumentation, a four-speed transmission, and all the modified bodywork that goes into making a G.T. 500 the distinctive-looking car it is. He also gets the beefed-up undercarriage, which includes stiffer springs, adjustable shocks, and modifications to front suspension to make it corner more flatly. On top of this, though, he must buy power brakes, power steering, integral rollbar with inertia-reel shoulder harnesses (the best we've ever seen), and a fold-down rear seat. This comes to a total of $264.77 before he ever starts ordering a radio, air conditioning, or whatever else he may choose. This is sort of like selling a car without seats and a steering wheel, but putting them on the car at the "buyer's request." All G.T. 500s are built with this batch of options, which are called "standard equipment," but which really are not.

For the base convertible price of $4,327.50, a Corvette buyer has all options still in front of him, with nothing mandatory except making the payments. The standard Corvette includes a 300hp, 327ci V-8, either a soft folding top or fiberglass lift-off type, and all instruments and gauges along with a three-speed all-synchro gearbox. From there the owner can choose any one of four optional engines and two four-speed manuals or a Powerglide transmission.

The primary function of an automobile, no matter what type it is, is to carry passengers wherever they may want to go. The G.T. 500 does this better than the Corvette. It has more room inside for people and packages, and will carry four adults for a short time or two children for a long time without complaints. The fold-down rear seat can become a parcel counter when only two are aboard, and the trunk bulkhead swings up for stowage of skis and the like.

The G.T. 500 is much easier to drive in traffic, as it's not as low as the Corvette, and it's not as ticklish to keep running at slow speeds. That big 435hp Vette engine likes to work hard, and when it's in bumper-to-bumper traffic, it objects.

The ride on city streets is much better in the G.T. 500. There's very little bumping around, whereas the position of the driver in the Corvette is very close to the rear wheels, and any rebound action from them is strongly noticed by the man at the wheel.

Vision in the two cars is almost equal, but still not excellent. We liked the '66 Shelby GTs for their rear-quarter windows, but construction differences on the '67 Mustang prevented their continuance on Shelby's version. Corvette convertibles have blind rear quarters, too, and the lift-off 'glass top isn't much better. The fastback model allows full vision.

We liked the interior layout of the G.T. 500, but it doesn't have the Maserati look of the Corvette. The Vette has all necessary instruments right in front of the driver, and all within a few inches reach. The Shelby car has the speedometer and tach right in front of the pilot, but the amp and oil-pressure gauges (Shelby additions) are positioned centrally below the radio. It's not hard to see them, but they're not as readable as in the Corvette, nor are they in a direct line of sight with the road.

The really impressive points of the G.T. 500's insides are its great-feeling wood steering wheel and the integral rollbar with inertia- reel shoulder harnesses. The wheel is one of the most comfortable we've ever had our paws on, with a smooth lacquered finish and a genuine "sporty" look. Shoulder harnesses can be cumbersome, and restrict the normal movements of the driver, but not so in the G.T. 500. They fit around you like suspenders. The inertia retractor in the rollbar allows slow movement, but quickly holds you against any sudden jerk or action. You learn to be leisurely when reaching for the cigarette lighter.