I had a new 157 pin ring gear installed on the flywheel
This statement implies that the same flywheel is in use but solely with a new ring gear; if mounted properly, and of correct engineering execution (CHINA?) then this is not the issue.
The difference between the two starter applications is whether what is often described as the "offset", this being a distance from the back of the block to the face of the ring gear, is either 3/4" (generally auto flexplates & 157T manual flywheels, 3 or 4 spd.) or 3/8" (generally 164T manual flywheels only) or the closest to as measured. To the best of my remembrance the only application for the shorter 3/8" offset is the 164T flywheels, most notably in O.E. installations with the cast iron bellhousings, which were configured with a boss cast inside the starter nose relief which precluded the possibility of mounting the longer 3/4" nose starter.
I tried a rebuilt starter, but it was 3/100” bigger than my original starter and didn’t mate up to the flex plate.
You have to open up the block plate opening with a die grinder in order to get the after market equivalent starter to work.
STOP! The only instances I've experienced that with O.E.M. components that it was necessary to "clean-up" the starter hole register would have been from some apparent damage inflicted upon the separator plate. After all, this is the "register" for the purpose of properly locating the starter in relation to the ring gear, and if modified, well as I'm often stating: your the new engineer! Yes, sometimes the alloy nose sections of the starter may require a little "clean-up", but if notably, greatly, obviously the wrong size, then take the offshore manufactured, piece of shyt back to where you found it! After all, if they couldn't that right, what else is not going to measure up!
Scott.