The Plymouth GTX (1967-1971) was introduced as the Belvedere GTX in 1967 by the Plymouth division to be a "gentleman's" muscle car.
Though completely redesigned for 1971, this was the final year for the GTX as a stand-alone model with only a manual transmission. Engine choices were 440 four-barrel, 440 with three two-barrels (Six Pack) , and 426 Hemi. Emission restrictions such as lower compression ratios and faster-acting choke operation lowered the base 440 output by 5 hp (3.7 kW), to 370 hp (280 kW). The 440 Six Pack was down to 385 hp (287 kW), but the Hemi was still rated at 425 hp (317 kW). Due partly to rising insurance rates on muscle cars, sales were low. There were less than 3000 units produced in '71. For 1972 through 1974 the GTX became an option package on the Road Runner.
Source: Wikipedia, 2011