Trans leaking in the pan gasket, stripped bolts, what size is the thread?

68 Plymouth Fury III, 383 with auto trans.
The PO had the trans fluid changed right before I bought it, Now that I've been putting some miles on it, it leaks pretty bad. A few of the bolts won't tighten. Spin and spin. No big deal, I'll put in some helicoils. So question is:

1. are the threads 3/8-16 or 5/16-18?
I dont want to remove one and make it worse. Like to order the helicoil kit now and do it all at once.

2. Any reason I can't or shouldn't put studs in there instead of bolts?...

Trans leaking in the pan gasket, stripped bolts, what size is the thread?
Author: Steve-o