Why handicap the SB with a 4 inch arm? I‘d build a 3.79 stroke (or maybe even a 3.65 stroke because I think they used to make that stroke length) and turn the small block 7500 rpm
......and still have more stroke, and less cubes than a 400bb. Why do you think the small block with a 3.79 stroke can out rev a 400 with a 3.38 stroke, and a bigger bore for more valve area and head flow?