I am quite certain your issue is not compression related. However, a good decarb is still a good idea, especially if there is carbon buildup. Pulling the exhaust plate will help determine this.
Forget your Item #2 on your list. Simply ensure that the points are set correctly, 020, or better yet using a timing fixture if you have one. Timing is fixed on these via the flywheel key.
I am still leaning to exhaust issues, be it carbon build up, damaged gasket, damaged plate, etc..., OR a carb set up/linc n sync issue.
Ill assume you set the needles on the water. Leaning out the high speed needle did not improve things? What is it currently set to? I find most of my Big Twins end up being about 1/4 to 1/2 turn out from seated once set.
To perform linc n sync, the carb butterflies should start to open when the pointer on the intake manifold lines up with the hash mark on the cam. NOT when the cam roller hits the hash mark, like on smaller hp motors of that vintage. To adjust, if incorrect, line up the pointer and the hash mark on the cam, loosen the cam screw with the oblong hole, and push in/pull out as needed, so that the throttle plate is just starting to open when these two items line up.
