[diverrick]

Years ago when I used to run fast, I built a 302 up from scratch. I used 351 heads on the 302 block. That gives you bigger valves as well as a bit more volume. Then I added pistons that had to be machined to fit. It all adds up to more power. Not sure why you go the other way and put 302 heads on a 351 though. Maybe for higher compressions?
Trying to remember if the 351 heads didn't have stronger valve springs as well.
One note though. If you are planning on pushing the engine, make sure you swap out the pressed in rocker studs for threaded no matter which head you use, or the pressed in ones will pull out, particularly with a high lift cam. BTDT

That motor tried to kill me more than once. I finally totalled the vehicle and told the insurance folks from my hospital bed to keep the motor. I am guessing some one got themselves one hell of a motor. Bottom end was balanced within 1/4 gram!
That thing spun up like an electric motor.