Port dimensions are exactly the same from 40P to 48P so you won't find any difference in gain etc between the two. The only difference is the intake flange area where the venturi resides. Either head will do fine if you want to keep the sloped port shape/design. The 48P venture is better suited for a better approach to fuel entry. The 40P with its narrow cathedral entry isn't as efficient as the 48P is in that single area.
