Another thought. Have you tightened the head bolts. If so, they may be overtightened, or tightened in a way that causes excess friction between piston/cylinder. Fox cases are on the thin side. Remove plug, rotate prop, loosen head bolts, tighten gradually in sequence, go from one bolt to it's opposite. Easy to distort head, easy to distort case. The aim is to tighten head so that piston rotates smoothly.
Having looked at the picture you supplied of your plane and mounting. (Finally realizing you posted it.) Could be a bad vibe developing on the profile. This happens. Profiles loosen up. Zap the fues/wing joint with CA, or epoxy, if you think that joint has loosened. Do it anyway. Put aluminum pads between engine and fues. Put pads on opposite side of fues so that the bolts and nuts have a firm surface so you can cinch tight. Fuel soaking, vibration of engine, can over time soften nose causing vibration issues. I have flown profiles that needed mounting bolts tightened every few flight. Tightened very tight. If I neglected to this, run, run, run, runaway. I mean tightened, by the way, with heft and crudeness. Not uncommon for the mounting bolts on that plane (hardened, socket, 4-40s) to pop now and then. But the method worked. Dan Banjock hisself has made up steel pads on a few models to distribute vibration stress.
Did you pressure test that tank by sealing up the feed tubes, dunking tank, and running lots of air pressure into tank. I have a dedicated fuel syringe that I push on hard as I can with tank submerged. Even a tiny trail of bubbles can reek havoc.