The Triumph Spitfire has a very large engine compartment area. This being the case, a variety of engines can be retrofitted into the car. Some engines, like a Ford V-8, are not direct bolt-ins. Not only will you have to modify the engine mounts, but all other components will have to be modified as well. These include suspension, brakes, and cooling and electrical systems.
Noted Triumph Spitfire author Calum E. Douglas of the Triumph Network states that a variety of engines will fit, from the stock engine to a Ford 427 Big Block. The main consideration is not the size, but the weight. The weight of a standard Triumph engine is 125 kg (275 lb). Bigger engines weigh more, such as the 427, weighing 318 kg (701 lb). This is more than twice the weight of the standard engine. You will have to weld in extra framing members, and install new suspension to handle the extra weight.
TR6 and GT6 Engine
The engine needing the least work to retrofit is the Triumph GT6, or the Triumph TR6. This is a six-cylinder engine, displacing 2 litres. Its rated horsepower was 104 hp in its final year of production, which was 1973. Many parts are available for this engine from aftermarket suppliers. This engine is almost a direct bolt-in. You will have to obtain other parts from a scrapped-out GT6, such as the cooling system radiator and associated hoses. This engine weighs about 74.8 kg (165 lb) more than the original engine, so you have to obtain suspension components from a GT6 or a TR6 to upgrade the suspension. Also, bear in mind the frame will have to be reinforced to be brought up to GT6 specifications.
Most American engines will fit inside the engine bay. For example, a Ford 3-liter V-6 will fit, but you have to do extensive modifications. First, the engine mounts will have to be welded on. Next, the radiator will have to be replaced with a larger unit. You will have to also change the transmission to a Ford unit, since the original transmission will not bolt up, nor will it handle the horsepower. The frame and suspension will have to be strengthened, to handle the extra weight. You are entering into the area of experimental vehicles, and a design engineer may be able to assist you with the necessary modifications.