When you are dealing with carb spacers it is best to try them out on a dyno to see what they do. I had one on a 468 that I was dyno testing, and this particular engine had some mixture distribution issues caused by a 2" open spacer. We tried some other combinations, finally ended up with a 1" four hole spacer on a 1" open spacer. This corrected all the mixture distribution issues and the engine picked up 40 hp. It depends what the engine likes, and I have no other explanation.
