I measured as best I could and the gap between header and pipe on my set-up is approximately 1-3/8". Hard to get any more precise than about +/-1/16" because the coupler tape, of course, is covering the gap.

Header lengths will vary, so this number isn't going to be the same unless you're using the Macs header for the OS 1.40RX that is of the same vintage as mine.

Bottom line, do whatever it takes to end up with the 27-1/2" plug to baffle distance.