How much press should there be when I press in the new bushings? And how much clearance when I ream them out?
I always tell people the first Model T parts they should buy are these...
...because they can save you a lot of grief, hassle, and dough. BUT, in this case neither the Ford manual nor the MTFCA Electrical System book answers your questions. I have no idea about the first question. For the second I'll guess .002 to .003. Maybe somebody who really knows for sure come up with an answer.
There are so many things to suggest...but I'll stay on topic to the specifics of your question. The large bushing at the bendix end of the starter is frequently in good enough shape to reuse. The final reamed size is an odd size (which I don't recall here and now...will check when I go home) and I have never seen it in print (it's not in the electrical book, that's for certain). Additionally, that bushing needs to be line reamed such that the brush housing and bendix bushing and brass bushing are all in a line...not something every Model T mechanic is equipped to do.
It's so easy to order a new bushing from the vendors, it's hard to believe what a bee's nest you've batted when you go to actually replace it.
If the old one can be reused, you can remove it, cut it off to install an oil seal, and reuse the old end for a thrust washer. Just be sure to rotate it slightly when reinstalling to make sure the oil hole does NOT line up with the casting, or the addition of the seal was for naught.
One item down and 5 to go (field coils, starter post, brush plate, babbit bushing, commutator).
Starter repair can be a humbling experience. Keep us apprised.