Its not just a simplistic question of Single vs Dual plane, but in the OP specific case,,a TINY dual plane vs proven Single plane and (most importantly) on a 408" with a decent cam making peak hp at 6000rpm.
It's not unreasonable to think a M1 would work considerably better on a 408" in every way WHEN compared to the LD4b. If MUUMUU already had a Performer RPM, that would change everything and I'd strongly side with keeping it as is but not with a LD4B. The LD4b is just too small for that size engine.
Also, I'd often suspect a good part of the "less than desirable street manors, slower 60’ & crap mileage" that the single plane gets the blame in misplaced. The majority of "issues" after a intake swap could easily be contributed to just poor tuning or most often ZERO tuning whatsoever. BUT add a good self-tuning EFI to the mix and again, that changes EVERYTHING! Again, my M1 with multi port EFI no such issues but it was tuned very well(A/F & timing). But I'd happily admit that when I had the SAME setup running on a 750 AFB, it ran worse in every way. Yeah, it was not the intakes fault lol.
