The stock bottom end can handle 1000 hp. The cranks are interchangable except the 4.8L, the rest are the same except the early 6.0L have a thicker/longer flange for the flywheel. The stock pistons won't handle too much abuse since they're hyperutechnics. The 6.0L block is a great start. Buy an Eagle 4" stroker crank and forged rods and you've got a 402 with stock bore or 408 with .03" oversized bores in the 6.0L block. Buy forged pistons to go with the rotating assy and you've got one stout engine.

Once you go into one of these engines you might as well spend your money wisely and do it up correctly. You can buy all the parts to build up a nice new 408 long block for about $5500.