I can only say that I’ve read a couple of race teams finding over 500 hp with the 2bbl single plane M1. The throttle body being a limitation to the intake is the only obvious problem I see starting with what throttle body your working with.
Opening up the area where the throttle body sits is easy enough. But after that, how that intake flows past the throttle body into the plenum and into the runners, down the runners into the head, I’ve never seen a slow bench report or any words on how it is.
Where is “Bel-Ray?”