I had a look at to buy a 73 bus today.
It had issues with going into 2nd from 1st or 3rd, and the clutch point is a lot further out than I would like. Could these be linked? Is this a gearbox or clutch about to shit itself? It is a 3 rib gearbox with a 2.0L.

If it is a gearbox issue, what will a replacement cost me?

Sounds to me like shift rod bushing and clutch adjustment needed

you will have to crawl under the bus and look for slop
the bus will have at least 2 bushings to deal with one just in front of the gear stick shift rod and another one further back along the shift rod
Also the reverse stop plate under the gear stick will be worn and maybe even the gear stick ball end can have issues as well
Also check the trans shift coupler at the nose cone of the trans for slop and or loose grub screw

Or it could be the bush on the hockey stick in the nose cone of the trans
But you will only find that after you pull the trans out
Also there is a bearing in the trans that is made of unobtainium as it an odd size and it's the one bearing that chews it's way into the aluminium
gear box housing and causes problem also
So in short .............welcome to the wonderful world of working on 50 year old dak daks

As for the clutch issues
could be a cable just about to let go as some of the wire strand have snapped
to much or not enough sag in Bowden tube at the trans
have a look and see if the earth strap is still connected from body to trans if not the clutch cable becomes the earth strap and that chops them out quick sticks
Maybe not enough lube on the cable and binding up with dirt

Anyhow I could go no, but its now way past my bed time
Best of luck

Could also be a bent or broke clutch fork / throw-out bearing or pressure plate had just about failed

engine oil or trans oil leak making a sticky mess also

3 rib boxes still cheap, get one $350 where 2L ones are $750 plus double if recoed amd more