My '78 Pace Car had a lot of looseness in the shift linkage. With the cable disconnected, I had 5/8" rotational travel at the outside connector on the TH 350 shift shaft. The problem was internal, so by dropping the pan I found that the large nut on the internal end of the shift shaft had back way off. The "rooster comb" (detent mechanism) was barely clinging to the shift shaft. I put it back up on its square lug and retightened the heck out of the 11/16" nut. Problem solved!
While I had it this far, I re-torqued the valve body bolts and everything else that I could put a socket on from below. I had already done a filter change with new fluid last fall, but didn't take the time to suck out the torque converter because the cooling lines are nearly impossible to get to at the radiator. I could have taken the chance of removing them at the tranny (risking breakage) and used some fittings to let the pump take out what it could, but I still wouldn't have gotten what was left in the cooler. This time, the "new" fluid and filter were polluted with the remnants of the old fluid from the converter and cooler, so another new filter and fluid were in order. At the very least, I now have approximately 75% new fluid in the system and all seems fine.
Hope this can be of some use to anyone who may have a loose internal nut on the shift shaft. It's a pretty simple fix. The nut won't fall off completely because it bottoms out against the valve body before it can do so, but the situation makes for a very sloppy shift mechanism. Also, since the park/neutral/start switch is in the console (not on the tranny), the darned thing started in reverse on me when I least expected it! Imagine my surprise!! I decided it was time to take action. Best regards to all!