It depends on the cating. There were some 396 blocks that were capable of going to 4.25 bore to make a 427...I have a friend that went this rout...but you need to make sure you have the thick casting first...sonic checking it is the only right way but a backyard way is pulling a freeze plug and looking in there...I read it on here something about whether or not a pencil would go between the cylinders I think it was.