[blue goats]

Welcome
I don't think stop leak will fix a rear main seal leak. You'll have to replace it with a neoprene one. The rope seals they sell today only work for awhile in my experience. Once the car sits for a while they leak. Regarding the trans , is it bad ? Changing transmissions is a personal choice for me. Some like overdrives some like just stock and some prefer powerglides for racing. As for the lead I really don't know. Has the car always ran with some sort of leaded gas previously? I've been told once you run unleaded thru the car for a couple of tankfulls the damage is already done, whatever damage that might occur has already occurred.
Hope this helps and welcome again
George