For what it's worth, I run the PowerMaster starter 9426 on my 572ci first gen Camaro. We have had no problems like what you described. We did remove the starters' axial shim that came installed out of the box. The axial shim was preventing the starter gear to engage the flexplate teeth properly. With shim removed, my gear now engages about 80% axially into the flexplate. It works perfectly like this. We also verified proper radial gear clearance and found good mesh without adding shims.
One final thought, we used the 9426 starter which allows all three starter bolt locations to be utilized instead of just two. Your 572 block should have all three tapped holes for this which provides a really good stable mount. And make sure to use knurled bolts from GM or equivalent to prevent starter movement.
Does this help?
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