I agree with all of the above. Spend the extra $$$ and get a better block if you want to take it to 427 cubes. Or, if you don't want to get a new block you could always stroke yours to 392 or 408.
I always put a good word in for these guys when someone is talking about stroker kits. They're shop is near me (but they will ship) and I know a lot of people that have used them and are very happy with their products & service.
www.dssracing.com