With those heads the should be no problem going well beyond 400 hp on a 400sb assuming your cam isnt stock type lift. just buy a new block for $100 and spend what ever it cost at your shop to bore it and rebuild the 400. or as road agent said you could use a 350 block, but theres more to it than just clearancing the pan rails. You would need new pistons, have to mains turned down(costly) and might as well have it balanced while your at it.