With the difference in ring tension you found, I believe I would have gone back in with a new set of surplus multifuel rings on the new pistons, especially with all the labor time you have in it. That's just my thought on the matter. Ring tension in a high compression engine is certainly important. My best guess is that those pistons and rings are for an 80- 90, or maybe even a 120 horse White tractor engine, which of course would have lower cylinder pressures, thus not needing as high of ring tension.
If you still have issues after proper break in, then there is your culprit.