You can shave the 390 pistons if they are thick enough. And if the block checks out to stay the same size. Or if not buy a off the shelf 410 piston. And yes you need to internally balance the rotating assy or change to an external balance flywheel or flexplate when using the 428 crank. Just be sure the 428 crank is indeed a 428 crank! Edge of #7 counterweight on the crank will have a 1U or 1UB. The stock 390 balancer can be used as well.

Do you have a Cougar? If so what year and etc.....?? I have a 68.