Those lower bolts are a reason why so many 350s get their reverser and transmission cases ruined. The factory puts them in super tight. Near impossible to get them that tight yourself again. So they loosen, the two locating dowels wobble, shafts and cases get ruined. Most 350s I look over these days are coming loose where the two cases join.
When we took apart 350s when near brand new - we'd heat the heads of the bolts cherry red so they'd stretch. Then later, we'd use new grade 8 or 9 bolts to put back together and tighten with chain-wrenches. We had to take apart a lot of nearly brand new 350C with blown reversers.
In regard to your knock. I suggested earlier you pull off your reverser filter and stick your hand inside to inspect the counter-shaft. You can reach in and put your hand right on it.
You are gong to have your hands full. Usually pulling a reverser is no easy job. Usually entails a few or more broken or stripped bolts. Have your side-frames been cracking where they bolt to the final drive clutch housings?