We have raced both over the years.......we now (Robbies Roadrunner) run 400 combo. Do not see a lot of flow difference between 906 and 452/346 Heads. BIG difference between small AVS and large thermoquad (really any thermoquad) that is legal on 400. I have never seen a 400 combo that is not FAST from start! I have seen quite a few 383's that the guys have struggled with, although I know a purple '70 383 that is real fast up north. Let me know if I can help on either one.
Racer7411@yahoo.com