Starter / Flexplate

I have a 1975 Mark IV 454 that I have just rebuilt, when I purchased it, it did not have a flexplate. From what I can find everything shows that it takes a 168 tooth flexplate. So I purchased 2 just to see if the flexplate is the problem. Funny thing is the one I bought local and the Pioneer brand from Summit are the same flexplate. My problem is when I activate the starter the gear comes out and does not disengage and go back when I unhook the power. I have 6 shims and they make no difference. It seems I have to back off both attachment bolts at least 1/4" and hold the back of the starter up to get it to release. I'm reading there is also a 153 tooth flexplate. Might anyone else have had this issue? Does anyone know if the less tooth flexplate is the solution to my dilemma? Thanks in advance as you guys are always great with knowledge and info.