I have to agree that something was way off on those Dyno numbers. Unless something was bad wrong with the qjet and something very right with the Holley there should not be that much of a difference. Did the OP say that he built the qjet to Cliff's recipe or that he had Cliff build it? I can't remember what intakes were used but the Holley would've required a spacer to mount on a spread bore intake. Might account for a few ponies. A lot of the time the first pull doesn't read as high as the second or third but usually not that much. I think a fairly well put together 455 with a good carb and good exhaust flow could possibly make 490 on a generous engine Dyno. I could be wrong.
The carb was a somewhat rare Holley spreadbore designed as a direct Qjet replacement (no longer available new from Holley), so no spacer. Pretty sure if they were worth 100hp, Holley wouldn't have discontinued them.
Carb ratings are based on 1.5in-Hg for 4bbl (3.0in-Hg 2bbl) pressure drop as a standard. It is possible on a dyno to see <1in-Hg for the engine. Which makes the effective flow rate for the carburetor smaller. You'd have to know the pressure drop during the run, to determine if the smaller Q-jet was a possible restriction. Still I wouldn't expect more than 10% between the two. And even if the tune was dead on for each, the most you would expect is roughly 10% difference (for exact same cfm carb) just due to lambda being off.
So, either it was: a combination of air/fuel + cfm difference; a dyno input value was fat-fingered between runs; the throttle linkage wasn't allowing 100% WOT; or (and I think the most likely) the airvalve on the Qjet wasn't opening properly.
