[blykins]

For sane horsepower levels, $4000-5000 blocks are not necessary.

The biggest disadvantage for going forced induction on something like this is that 1. You don't need it for the horsepower levels that you're looking for (it's not a small block) 2. The cost. You're probably going to double, if not triple, your engine budget with two turbos/superchargers.

Getting 800hp out of a P51 headed 557 isn't terribly hard at all.

I still don't know why you have to have 800hp.....that's not streetable. Anything above 600hp in a street car is pretty much a dice throw anyway. You would never be able to take advantage of a wide open throttle situation.

IMO, 700-800 is for drag racing, not street driving.

You could build a 600-625hp 460 for pennies compared to what you're trying to plan.