After seeing cap walk on my flat tappet, 906 head motor with ly rods and light Venolia pistons (4.5 stroke) at only 5700 rpm max, I will go as light as possible on a deal like this. I am betting that the performance difference would be very marginal between the two, but stress on the stock block is what I would worry about first. If it is going to be a very low rpm deal, say 5200 or less, then I wouldn't worry much. But if you are looking to push 600 hp, then I would.