M1 or Port-Matched LD4B on 408

If the LD4B is making 523 HP and 522 ft lbs of torque, KEEP IT! My 416" motor has more Ricky Racer go-fast parts but makes less power and torque than your milder 408.

Switching to the single plane will essentially move the usable RPM band up which IMO makes little sense for street driving and road courses. Drag racing, yes but because that's only WOT. Can't vouch for your driving style but you generally don't stay at WOT for long periods on the street or road courses. Having more torque available at a given RPM would be of more benefit.

Staying with the LD or any dual plane will theoretically keep the torque peak in a more usable RPM range. Maybe an LD340 or as suggested already an RPM Air Gap would be what you want? The AG gives up very little on the top end, probably be a better choice over an M1.

Torque benefits you most coming out of turns, the car will accelerate quicker at a lower rpm and would be easier to rev match. Don't think you want to slow for a turn then alternately have to zing it to 7 grand to get to where the torque starts coming on.

Sniper - I had mistakenly thought that EFI on a dual plane was a no-go becuase of fuel distribution issues but I was corrected on that by a well-known builder. If the combo does OK with the dual plane and a carb, it will be fine with EFI.