Sounds more like a internal linkage issue than a gear issue....
Please post what you learn. I put a five speed Tremec in my 64 Vette. So far, I'm happy with it but it only has around 500 miles. I only put about 300 HP to it but I can't see your stock 383 being too much for yours. I'm a little surprised your fluid was dark. My unit required ATF type fluid but hard to imagine there being enough heat to darken it. By the way, was there any warning signs before failure? I ask only because I notice my second gear is just a hair tighter than the other gears.
Are u using synthetic oil in trans? Read this is causing problems.