I asked a question similar to this on the gaming forum. That might be where you want to go. A year ago i would have had the perfect answer for you : Motor City Online. I still haven't been able to find a game as awesome as that. You could pick literally EVERY part of your car, assuming it was compatible. You could choose the block, the cam, which pistons, which crank, the radiator and cooling system, the tires, tranny, rearend, heads, etc. I still have the game installed, but they shut it down, and nobody has figured how to crack it into single player to make use of their parts database
If anyone does know of a similar game, please let me know as well.