okay here's my bit on this.

mechanicals - the master cylinder on the 71-? bus mounts on the front axle
beam, whereas the earlier (all drum) mc mounts up on the body. 
You could fabricate a mount (weld a plate with holes in the 'right' places)
for the mc booster assembly on an earlier axle.
If you can find one, an easier solution is to get a later model axle beam
with all the rest of the brake stuff you will need.

I have put a 71 axle beam on a 70 bus and it does fit btw. 

hydraulics - the mc for the disc/drum and the all drum systems are going to
be quite different - i think you would want to use the 'correct' mc
in order to get reasonable braking. 

I think this is a big project requiring access to a lot of parts - for
instance the brake pedal lever is mounted differently on the disc and drum
brake models (reflecting the different mc mounting points) so more work tbd.

wont even mention (ok i lied) the brake pressure regulator

on the pther hand its possible you could just swap the drum brakes with
disc brakes on the front and it would brake reasonably well. I would NOT
be willing to go to all that trouble for the chance that this would be the case., however......

speaking of axles, i happen to have a 'good' 71 axle with drums on it
(from the old 70) in my garage - will junk it before winter.
Its in upstate ny - free for the taking - no i dont deliver
(okay maybe i could drop it in rochester, syracuse, albany, on one of my trips
through but no callers from BC okay?????

:-)