The main differences would be head gaskets, freeze plugs and if its for automotive, the cam profile. This is assuming you are only talking about a long block, not all the accessrories.

BT :cool:

I had a customers boat that had a 454 making approximately 500 HP. I ran the boat on GPS, then installed a set of Stainless Marine manifolds and retested within a few hours at the same place, same conditions. It had a noticable increase in midrange and gained 2 MPH top end on GPS.