how far can a 390 be bored safely? I have a 63 block. I am trying to find a 428 crank and rods I can afford. I also have the 332 heads which I hear increase compression due to smaller chambers. Would they be a good idea if I stroked the motor? Would the 63 390 heads be better? I am planning on using flat top pistons, an RV cam, but I have heard that too much compression causes problems running on pump gas. I don't want that. any help will be greatly appreciated