Here are my options: Rebuild the current tranny. $100 for a rebuild kit, $130 for a torque converter. I have never done this before, so I am reluctant to do it.
A friend of mine has a late seventies 350 trans from a Z28, no lockup torque converter, just a stock one, correct length tailshaft. He thinks it's in good condition. Cost-free.
Take it to a local transmission place. $700 includes torque converter, free towing and 12 month warranty.
Hopefully, the rear main seal is one piece. Then he could replace it at the same time.