jimmuller 

The GT6, TR6, and Spitfire are intimately related. The engine was developed as a "small car" 908cc (or maybe 803cc?) 4-cylinder and gradually enlarged to 1200cc. The 1200cc version was expanded by 2 cylinders to make 1800 for the sport sedan Vitesse, and the Spitfire version went from 1200 to 1300 to 1500. The GT6 was originally intended to have the 1800 but before production started the engine was bored out to 2000cc, so it got an engine boost before birth. For the TR6 they used a longer crank to make it 2500cc.

As for car models, the Spitfire was designed as a roadster based on the Herald sedan and came out in '63. In the meantime the designer Michelotti developed a fastback version but it was too heavy for the 4-cylinder engine. It languished for a year or so but was used as the mold for the fiberglass fastback Spitfires that Triumph ran at LeMans in '64 and '65. (Actual footage of one of those races, '65 I think, was used in the French film A Man and A Woman.) The publicity success at LeMans encouraged Triumph to produce the GT6 but with the 6-cylinder engine.

The TR6 was based on the traditional TR3/4 chassis but by the mid-60's Triumph knew they needed a bigger engine, and thus prepared the 2500cc 6-cylinder. However they had no capacity to build a new body so for '68 they put the 2500cc with Lucas fuel injection into the TR4 body and named it the TR5. But for N Amer. they worried about no one understanding fuel injection or its poorer gas mileage, so they went back to dual Zenith-Stromberg carbs and named it the TR250. The TR5/250 lasted only a year because by '69 they had contracted Karmann to design and build a new body. That's one reason the TR6 looked so different, a German design vs. the previous Italian design. The TR6 lasted through 1976. The GT6 lasted from '67 through '73, with upgrades in '69, and a new body in '71. The Spitfire went through five version and lasted from '63 to '80.