The real problem is wear on the countershaft, as it uses thrust washers, not tapered bearings, and has no center support between input and output shaft, like a modern trans, so they get loose with big power or towing in overdrive. Use gear oil, change it often, don't beat on it in overdrive, and it will last a long time. Start with new thrust washers, bearings, needles, countershaft pin, and shim any excess endplay out of the cluster gear and run it. Just remember it was designed for a light 6cyl car so.... one last thing, check the ends of the cluster, may be worn from thrust, or wiped washers and metal.