Well, I should say I was 20 when that happened ('90). I pulled the trans, and stuck it under a bench, and replaced it with a 4sp. (car was a 383 with some nitrous). Years later I sold all my cores (part of my divorce), and about 2 years ago the guy that bought them called me and told me what he had found, and how had I done it?
I drove the car daily, and when that happened I drove it another couple days with it getting progressively worse. He's never seen one do that, I've never seen on do that, and between us if we haven't built 100 transmissions and taken more apart for parts I'd be surprised.
Keep driving it, but liek the Dr. ays "if it get's worse, [replace it]".