if you have a running engine then i would just do a cam change and slap on a supercharger. 425hp should be pretty easy. if your going to add new pistons, then figure reconditioned rods, turned crank, and bore and hone the block. your already looking at about $2500 worth of work when you include the heads. a supercharger isn't much more expensive and has alot of potential. also you can get a used setup for a small block chevy off of ebay, if you don't mind doing some fabricating