I believe only the NMCA nostalgia super stock classes allows single carb combos. Victory and NSS Inc. only allow dual carbs. Not sure about the other groups. The Modman from Indy seems to work OK, a lot of guys used to run the older edelbrock 2x4 manifolds. And as mentioned you pick an index, so if its a few tenths off, no big deal. I ran a 63 factory cross ram for several years, it was 4 tenths slower than a edelbrock victor Jr , I modified to run 2 carbs. Rules state you must run a as cast manifold but are allowed to modify the top. I have access to a spool gun to weld alumninum, I got $350 in my intake and two used 750 federal mogul AFB carbs off ebay for $75 each, car has run a best of 9.65 @ 140mph. The best I got the cross ram to run was 10.01 & 138mph, same engine only manifold change, the same carbs on both manifolds.