Having worked with both brands in the past, I would go with the Sealed Power (Speed Pro if you can) pistons. They seemed to make more power than the Keith Black's. Same engine... Same set up... Same internals. Only difference was the pistons. I did manage to find out the difference too... the Sealed Power pistons had a little more wrist pin off set than the Keith Black's. Now, that being said, I was working on a 5.0 and not a 4.0... Might not be the same with your project.