compression tester all read different due to hose length, adaptor volume, etc. I wouldn't worry about the number as much as the same number on the center and mag, pto will be less due to larger volume chamber on the head. I don't think compression is the problem.
I also wouldn't start just throwing different radical difference helix at it and hoping to hit a bullseye, as your going to have to keep changing out the center weight to adjust for rpm. You tune the front clutch for rpm, not the rear clutch! the helix merely tailors the shift. Nothing wrong at all with the 51/37 if you know for a fact its a 51/37. Where most guys get into trouble is they buy" coded helix" from bender,hauck, maxx all used codes so the average person couldn't just go and copy a clutch kit easily. Guys think that's the angles because someone posted a list. I can tell you the list I have seen posted has a lot of wrong information on it! also one brand helix will shift different from another brand due to the radius cut on the ramp. For example if you had a known 50/40 from hauck and a 50/40 adavntedge, youll see 100's of rpm difference in them and a way different shift curve!