Quick wins....
There's a linkage about six inches forward of the gearbox- this chap-
that often wears out/perishes, so may need replacing. keep the metal cage off the old one though, they seem much sturdier and the pin that goes through the bushes feels tighter. You'll know what I mean when you see it. It's an easy swap.
there's also a bush up front... right where the gearstick fits into the main shift rod, there's a little bush inside the rod, that slips onto a pin. You'll need to take the front section of the shift rod out for this one. there's a little bolt with a bit of wire on it near where the shift rod comes out of the big guide tube that runs down the van. remove the rear coupling as above, then that bolt, then (or before, I don't care) unbolt the gear lever and pull that out. then you should be able to remove the front shift rod section, and replace the little bush inside it.
Here's a pic of the front shift rod and it's bush-
Then you can install a "quick shift" kit- sounds racy, but they really help. they're only a fiver from places like GSF. People say that you shouldn't use the plastic ones, but i've not had a problem with mine in the last 40k. that goes both underneath your gearlever and slides onto it before the retainer plate- this looks like a pretty good write-up on the subject
http://batvolks.blogspot.co.uk/2009/03/quick-shift-kit-installation-on-72-bus.html
And that's as much as you can do without taking the engine and gearbox out. when you're refitting the gear lever, do the two nuts up so that you can just about move the metal cover, then pop the clutch down and try to get all the gears. the cover usually moves itself into place when you do that and then you can nip it up- you might need to wiggle it about though.
if you wanted to do a COMPLETE overhaul of the bushes, then as I say, engine and gearbox out, main shiftrod out, and there are some shuttlecock shaped bushes on it (they're on the photo above, arrowed). they're easy enough to replace but an arse to get to, so try the easy things first.