Pre-04 isn't impossible to find, but getting harder. Just have to know where to look.
You'll want at least an MT643 or 2000 (MD3060 would be nice) transmission, at least a Cummins 8.3 or Caterpillar 3126. Navistar DT466 (E), Caterpillar 3208 are nice alternatives. A post-04 gasser could be worth a look if 6.8L V10 or 8.1 Vortec-equipped, but must be dual-fuel if it is propane-equipped (some are propane only), and will be thirsty.
"Cheese Wagon" <email@example.com>
Former owner - 1989 Ford B700 64-pass Blue Bird (Rest In Peace, Cheese Wagon)