Are you building the motor yourself? You lookin for a crate motor with a warranty?
I'd say a 496 is a nice budget friendly big block that can easily put out a streetable 600-650hp on pump gas.
For some extra thunder (and $) just go for a 540 or 572 and really make that thing scoot.
And budgets are key as you probably know, but you probably also know they are usually blown up about half way thru the build!
Do you have a power goal in mind?