I just went thru the manual but can't find the specs for the original tire size. Ed -
The OEM stock tire size for the 4106 was 11.00x20 tube-type BIAS PLY on split rims that turned 495 revs per mile.
Some Greyhound divisions back then spec'd 19" wheels/tires to reduce theft, others spec'd 22.5s - but that's ONLY Greyhound. Most other carriers simply opted for the stock size.
20" wheels & tires are almost impossible to find nowadays, everything is either 22.5" or 24.5." Ditto for tube-type tires, let alone bias-ply construction.
As I said in my earlier post above, look for tires that turn as close to 495 revs per mile as possible for OEM performance with your 4-spd manual. 22.5" or 24.5" rims, doesn't matter except that 22.5's are the most common. Just stick to the 495/mile number for best results.
FWIW & HTH. . .