The right spacer can make a huge difference,when you retune for it.
When I used Vic's dialed in Pro Systems and Wilson tapered spacer on my 421,my dyno's A/F fuel numbers showed it was real close to where it should be. Taking the tapered spacer off really screwed up the A/F ratio.
Same for QF 950 and open spacer,but to a lesser degree.
Each intake/carb/spacer combination acts different and should be tuned to get the best results
Keith