As planned, Carroll Shelby also developed a race-ready GT 350R with the same engine as competition Cobra 289s. That meant a nominal 350 bhp -- an outstanding 1.21 bhp per cubic inch. To minimize weight, the gearbox got an aluminum case and the interior was stripped down to a single racing seat with safety harness, plus protective roll bar.
Competition tires and super-duty suspension were specified, too. The front bumper was replaced by a fiberglass air dam with a large central slot for feeding in extra air. A few GT 350Rs were built with all-disc brakes, 400-bhp 289s, and wide tires under flared fenders.
As if all that weren't enough, Shelby devised a special GT 350H for Hertz Rent-A-Car, which ordered 936 examples. All carried Ford's three-speed Select-Shift Cruise-O-Matic transmission and black paint set off by gold stripes. Hertz rented them for $17 a day and 17 cents a mile. Some customers violated their contracts by racing the cars, but probably not as many as once thought. In any case, Hertz lost money on the venture and bailed out after one year.
In all, Shelby built 562 GT 350s for '65 and another 2378 to '66 specs, including R-model racers and Hertz cars, plus six prototype '66 convertibles. That was good production for such a specialized machine, but profit-minded Ford wanted far more. As a result, the original Shelby concept began to be watered down.
Ford bowed a heavier, restyled ponycar for 1967 with a first-time 390-cid V-8 big-block option. Typical of the man, Carroll went one better by offering Dearborn's new 428-cid V-8 for a second Shelby Mustang, the GT 500. Horsepower was conservatively advertised at 335, mainly so insurance companies wouldn't worry, but was closer to 400 actual.
The GT 350 returned with its previous power rating, but the true figure was now under 300 because the original steel-tube headers were eliminated to satisfy noise regulations. Both Shelbys sported a longer, more-aggressive new fiberglass nose, crisply clipped "Kamm" tail with prominent spoiler, and other appearance departures from regular Mustangs, plus small chassis refinements.
Interiors gained a large black-finish roll bar with built-in inertia-reel seatbelts. Shelby built 3225 of his '67 GTs, which sold for around $4000, down some $500 from the 1965-66 cars that were themselves incredible high-performance buys.
The 350 and 500 returned for '68 in convertible as well as fastback form, but all were somewhat less special than the '67s. Interiors, for instance, were stock Mustang save a console-mounted ammeter and oil-pressure gauge, and there were cushy new options like air conditioning, power steering, and automatic transmission. Styling was modified via a wider hood scoop and wide taillights with sequential turn signals (lifted from the Mercury Cougar).
The new convertibles listed about $100 above comparable fastbacks. At midseason, the GT 500 became the GT 500KR -- "King of the Road" -- denoting Ford's latest 428 "Cobra Jet" engine with jumbo ports and a new intake manifold fed by a huge four-barrel Holley carb. Fast¬backs rose to $4117 for the 350, $4317 for the 500, and $4473 for the KR. The costliest '68 Shelby was the KR ragtop, at $4594.
With Ford now calling the shots, the '69 Shelbys became even more like that year's fully redesigned stock Mustangs. Styling remained distinctive but was busier, with a big loop bumper/grille, scoops and ducts most everywhere, and reflective tape stripes midway up the flanks. GT 350s were demoted to Ford's new 351 "Cleveland" V-8 with 290 bhp; GT 500s, no longer KRs, stayed with the Cobra Jet, still at a nominal 335 bhp.
But after just 3150 of the '69s, plus 636 leftovers sold as 1970 models, Shelby and Iacocca agreed to end the Mustang GT program in the face of blossoming government regulations, spiraling insurance rates (the cars' accident record was staggering), and sales interference from hot new production Mustangs like the Mach I and Boss 302. Like the Cobras, however, these Shelbys quickly became high-priced, highly sought after collector cars.