[Idhomustang]

How large of an engine can be put in a 64-66 Mustang with little to no modifications?

The early cars were designed for the then new 289, as we all know 64-65. The early Cars 64-65 should remain 289 or small based windsor cars, the shock towers are closer on them then the 66, which is the first year that I would recommend implanting the larger windsor into.
If you do go ahead and swap in a 351 W try if possible to a set of early hp ex. manifolds, or a good set of quality headers that fit snug, that will help. As we all know the 351- can be built up to about 470 cubes right now using a Ford Motorsport block, but if your like me they cost way to much money unless one falls from the sky,undamaged I'm likely to continue to build motors with the cast iron 70- and later blocks.
The early big block cars you read about and see, once in a great while were factory drag cars or other wise, modified for big blocks, by widening the shock towers. I owned at 67 Gt with a 390 and it was pretty easy to work on,all things considered. My younger brother had a 428 car of the small year, both coupes. I wish I had kept both of these cars I let far to many cherries slip though the ole fingers..
The small 302 based motor can be stroked and bored to about 350 or so cubes if you really wanted too. Probe has kits that make the possible, with the right heads and cam, these can be very powerful motors and would serve any early model owners needs. And if you are not good at building motors you can install one of Fords Gt-40 motors rated at 320 horses, and make the conversion to fuel injection..