Hi new-velle,
if your getting 506 ci with a 4.25 crank, then you must have a 4.35 bore. What kind of block are you gonna use? I've read that a standard 454 with a 4.25 bore can only safely be taken .060 over. The tall deck truck blocks can be taken .125 over, so you could safely get the 4.35 bore with the truck block. Or are you going with a bow-tie block?