40 horse difference sounds outrageous to me, never seen anything like that swapping different intakes on small block mopars over the years.
single plane to dual, little single to big single, Indy to victor, never even a whole tenth on anything I have done.
Can only speak to what I have personally seen on our dyno's. At 100 engines a day, and 10 dyno cells I've seen lots of surprises. We have a 427 sb ford that does near the same thing. Makes 485 with a short Weiand, 521 with a Victor.
Now our SBC engines for whatever reason are less picky. Anything under 400 cid only vary a few ponies from a non airgap RPM, clear up to a Victor Jr.
All about the combo I guess. Can't say what happened on one engine translates to all. Otherwise dominator carbs would pick up 50 hp on 318's.