A833 Transmission

AJ/FormS said
Some play at the input, by pusking it sideways ... is normal. The ball bearing that supports it has a tiny bit of axial play
that amplifies out to serious sideways motion, at the end of the input shaft.
Damaged "splines" on the front end of the output shaft arev no big deal.
The clutch teeth on the gears themselves are easy to repair, also no big deal.
The pilot on the front of the M/S can also be repaired
About the only parts in that trans that need to be periodically changed are the brass synchronizer rings, and occasionally, the cluster-pin. And bearings of course. But if you start grinding gears, or have to hold it in a gear because it likes to jump out, well then, before long, yur looking at forks and sliders

BTW
I've never heard an A833 "thumping like a disco" so that will be interesting to see what caused it. I'd be more inclined to think that was in the clutch. But you said;

so I guess we'll see.
BTW-2
I hated pulling the trans out on the floor when I was 17,
and 50 years later, I know I would still hate it,lol.

I have a thumping noise, the tempo increases or decreases with speed.

Clutch engaged, disengaged, engine on / engine off, gear engaged / disengaged same noise. Vibration in shifter. Stereoscope has noise at transmission output.

Gets worse after shifting hard, calms down a bit after. The previous to the previous owner had pulled the transmission pulled twice (pulled the trany down once (warranty), replaced the throw out bearing etc the other time), drive shaft balanced and the rear end pulled and checked.

Still had vibration and noisy after. I have replaced the rotors (rubbing on calipers) and changed the rear end to a 373 posi (it was a 323 open). I found the the shifter was not set up right, the reverse rod was way out. It seemed better after, I changed the oil and thought I maybe had it, but I could not test drive as it was winter. Had one test drive last week, no power, changed the carb and lots of power but the noise got 10x worse... after hard shifts.