And undercutting should never be done with a Dremel, right?! But that's all that's left these days. It's the stuff of nightmares.
I never thought about a demel...Im fortunate in having a lathe in the work shop...
There cant be many old starters that have not been rebuilt or used older armitures.. therefore IF these companyies where reliable, then where required these armitures would already be under cut... in saying that, it is my experiance over the last 20/25 yrs, the alternator have not been tested on a growler...or even got a growler (as specified in the AC delcome electrical service Manual) a huge reason in its self for the terrible reputation GM starters have.
I also note here, AC Delco list hundreds of starters, with diffenent min / max rpm , current specs/ alternator specs....or even if Auto sparkies/ rebuilds are even awear of this....gab a random, case, a freild coil , an alternator and simply mix and match.
So get a heavy duty armiture with a std feild coil...
My thought is this...If one KNOWS their starter is 100% orginal, NEVER been opened up, yes rebuild it..and under cut....check on a growler.. and run intials (not confused with idle) advances over over 12 degs .....if not go after market with solid feild magnets or geared units