Mike,
The 65/66 Mustangs would require quite a bit of slashing and work. Shock towers are close and would have to be cut or modified among other things. However if this is just to be a drag car and not street driven, you can do it. I think the motor mounts would have to be re-done also and probably some extra bracing as those cars weren't designed for all the torque and power of the big block.
Ron
