Probably a few swap meets left up there, always tons of spacers there, maybe even a 1/2" spacer would do it. $5-$10 is typical, sometimes less.
I mounted my 4bbl sideways but the other way, so the primaries are furthest from the head to give a little longer/more equal runner lengths. Jammed a big tall post up next to the head so that's where the hard linkage makes the 90 turn, no cables.
The 390 Holley has SO much better manners than the Rochester Monojet you will be very happy when done.
