Stock heads/rotating assembly, hot cam/vlave springs- how far can you go?

You could try milling the plenum divider out of the MP dual plane and putting a one in spacer under the carb first, but it's still not going to match the RPM. Edelbrock really did put a lot of R&D into the small block Chrysler RPM intake, it's pretty safe to say it's the best dual plane ever made for the A type engine. With 4.10 gears, I myself would look for a good LA single plane and convert it to the Magnum heads and block the exhaust crossover. The top contenders for the street would be Weiand 7545 Xcellerator or Edelbrock Torker II (Too bad Holley didn't make a Street Dominator with 340/360 sized ports).