The earliest of the 1968 models were available only as 2-door pillared coupes (with a B-pillar between the front and rear windows), but later in the model year a 2-door "hardtop" model (sans pillar) was offered. The Road Runner of 1968-1970 was based on the Belvedere, while the GTX was based on the Sport Satellite, a car with higher level trim and slight differences in the grilles and taillights.
The standard engine was an exclusive 383 cu in (6.3 L) "Roadrunner" V8 engine rated at 335 bhp (250 kW) and 425 lb·ft (576 N·m) of torque. Its extra 5 hp (4 kW) rating was the result of using the radical cam from the 440 Super Commando and a .25 raise in compression to 10.5:1 (vs. 10.25:1 with the 330 hp (246 kW) 383). When air conditioning was ordered, the cars received the 330 hp (246 kW) version, as the radical cam specs of the 335 bhp (250 kW) version did not create enough vacuum to accommodate air conditioning. A $714 option was the 426 CID Hemi rated at 425 bhp (317 kW) and 490 lb·ft (664 N·m) of torque.
The standard equipment transmission was a 4-speed manual with floor shifter and the three-speed TorqueFlite automatic was optional. Early four-speed 1968 Road Runners featured Inland shifters, which were replaced by Hurst shifters during the course of the model year.
Source: Wikipedia (original)