Comparing the VF-1A,D,S,VT-1/VE-1 is pretty much like comparing the Ford Mustang 6 cylinder, GT, Saleen, and the Shelby GT-500. They're all pretty much the same except with slight or major differences.
But all in all this thing comes down to interchangable parts. I'm sure that it would've cost Ford (or any another car company) a hell of a lot more money to design a brand new model than to slightly change an already existing model for a less or more powerful engine.