Here's a tid bit on the 215 from Wik page on Triumphs.

The TR8 did not use Triumph's own single overhead cam V8 as found in the Stag due to an unreliable service record but instead shared its Rover V8 engine with the Rover SD1. The engine itself was derived from an early 1960s Buick/Oldsmobile all-aluminum 215 cu in (3.5 L) V8 unit that Rover acquired from General Motors in the mid-60s. History has shown this engine to be an extremely reliable, flexible, and robust powerplant. TR8s were initially fitted with twin Zenith-Stromberg carburettors. However, all 1980 models sold in California, all 1981 models (of which only 352 were produced including 20 cars for the UK market) and all 1982 models (of which all 69 went to Canada) featured a Bosch L-jetronic fuel injection system with a specially designed Lucas fuel injection computer (ECU). The carburettor model was rated at 133 bhp (99 kW; 135 PS) (at around 5000 rpm) and the fuel injected at either 137^[3] or possibly 148 bhp^[4] delivering 060 mph times in the low 8 seconds.^[5] Other differences between the TR8 and TR7 are upgraded brakes, revised axle ratio (3.08:1 on the TR8), battery moved to the boot, alloy wheels, leather steering wheel and a few minor trim changes.

Rare for certain; that is the first Viva GT I have heard of since 1971 and only the second one ever!

I think that in the U.K around 1974 they sold a Firenza version of the GT called the Vauxhall Magnum.