The wheel cylinders aren't that expensive. The shoes aren't that bad to get relined if need, though in my experience it's the wheel cylinders that are bad and the shoes have lots of life left in them. If you take the time and go through all six wheels and fix everything up and clean up and lube the adjusters etc I have found that the M211/135 serries trucks will stop on a dime and give you 5 cents change. Normally I say 9 cents, but they are big trucks. The other area that I have seen to be problematic is the master cylinder and the air pack.
When I had my M211 I redid all the wheel cylinders, a new master and rebuilt air pack and cleaned and adjusted the shoes and boy would that thing stop!
I think you'll be better off and less out of pocket to do this than to try and fab up disk breaks. I think I had maybe $200 total in my truck and that was 9 or 10 years ago.....so maybe at todays prices you'll be closer to $300. I think a disk conversion is going to be a lot more than that and will be a real PITA with the fabricatiing that will need to be done.
