Busch cars use V8's, I think they always have.
The SVO (Special Vehicle Operations, the precursor to SVT) made a hi-po Mustang, but even then it was a turbocahrged inline 4.
You can buy the small-block Chevy motors from a Nextel Cup car, a Busch car, or even a Craftsman truck, if you want to pay a good penny for one. The Nextel cars get about 800 HP, Busch around 725-750, and less then 700 for the trucks, although the motors in the Busch and trucks can be upped to 800 easily, they are detuned through the rules. I've seen a few places in the past online where you could buy used motors, although they were detuned to about 500-600 HP for the street, although I can't seem to find any of those places now.
Really, you'd be better off and cheaper buying a used motor from a junkyard or local paper, rebuild it, and pump it up to the HP level you want. You could even buy a long block crate motor that'll make a certian HP number. If you don't mind using a Chevy motor, the options are nearly limitless....quite a bit more limited if you want to stick with Ford.