1' spacer under carb

Engine masters just did a spacer test, open and four hole on a two plane, and open and four hole on a single plane. Lots of info there, but in summary, open spacer on the dual plane gave better results, four hole on a single plane worked best.
There are lots of variations of spacer, and countless combinations to put them on. As 273 said, just something you have to try, to know what works for you.
IQ52 tested one of his engines a few years back on his dyno, and reported the results here. He showed a fifteen hp improvement on his combo with the right $20 spacer. Hard to beat that kind of return for the money.