Nelson engineering in the USA has developed what it claims to be ther world's biggest Harley-alternative powerplant, by mating two of the giant V-twins to create a V4.
The machine uses two Evo series H-D engines for a combined 214 cubic inches of capacity (3.5 litres) and a claimed 240 horses.
Nelson says the powerplant goes in with relatively little modification and, as the pic shows, the installation looks very tidy.
See this link for more detail.