It seems there's always something special coming out of Unique Performance in Farmers Branch, Texas.
Several months ago, we spied a yellow '68 convertible in the back of its shop. It was definitely unique with "GT500SR" in the lower rocker-panel stripes on the front fenders. We knew right away the 500 designation meant big-block in Shelby lingo, and we have come to know that SR stands for street/race in Unique's continuation business.
So, what we were staring at had to be a really hot big-block Shelby continuation convertible. Obviously, Unique was looking back at Shelby history and moving up through the model years to continue other Shelby lines of the '65-'70 era.
Later, Unique's president Doug Hasty told us, "Our GT500SR is based on the requests of customers. They asked us to look at doing something similar to what we've done with the 350SR and the 500E." Shelby didn't come out with a convertible until the '68 model year. Thus, the GT500SR is catalogued as a '68, although '67 models may be used because the basic body style is the same.
To seasoned collectors, the sight of an R-model-style fiberglass front apron replacing the stock bumper is an odd sight on a '68, but it certainly pushes across the high-performance message. The SR in the 500 series is a no-nonsense build. Its performance is the car's entrance into the Shelby legacy, and we're talking 21st-century technology here. The 500 designation means big-block. Carroll would have it no other way than his own aluminum-block 427, an ageless torque monster based on the vintage FE design, or as Hasty put it, "This is the way Carroll would have built an SR in 1968 if he had the ingredients we have today."
Luckily, the hood was open when I first spied the car in the shop. The FE-series Shelby 427 aluminum big-block looked downright wicked with the DC&O electronic fuel injection. By design, it's easy to mistake this setup for a rack of Webers, but old-time carburetors can be fussy, especially on the street. Modern fuel injection allows the tuner to make the best of air/fuel ratios for maximum horsepower.
How hot is the 427 fuelie? The spec list Unique handed us reads 525 hp. Apparently, this number is conservative. It's also the rating with a carbureted 427, which is the base engine. With either carbs or the optional fuel injection, "You're looking at every bit of 600-625 horses," Hasty tells us.
Planting this amount of power to the rear wheels of a '68 Mustang, which is rather flimsy in its stock configuration, requires a considerable amount of modification. Like every Unique-built continuation Mustang, the GT500SR is drastically modified. The suspension is similar to the Eleanor and the Super Snake. Factory stamped-steel control arms won't stand up, so Unique replaces them with TIG-welded tubular uppers and lowers featuring 4130 chrome-moly rod ends and Teflon inserts. Steering is modern rack-and-pinion.
Although an original 9-inch rearend would be tough enough, it doesn't fit Unique's coilover rear suspension that features laid-down springs and shocks as well as upgraded leaves that fit tires as big as 315/35s. The Currie Tru-Track differential is basically a 9-inch highlighted by a big-bearing housing, a short yoke, and a nodular case capable of planting 900 hp to the pavement.
Making mega-horsepower is one thing. Utilizing mega-horsepower in a 40-year-old Mustang is quite another. No manual four-speed from the Ford arsenal of the '60s could take 600 horses, so Unique engineered new motor mounts and a transmission crossmember to retrofit a Tremec TKO 500 five-speed.