A more detailed pic of the rear wheel would confirm it, but is seems that they are indeed the RH-5's. If there's about a 2" wide raised flat area in the barrel area of the wheel, that will confirm your suspicions. The multi-piece wheels for 20" tube type tires with a split ring forming one bead, or the 3-piece ones with a full circle bead ring and a lock ring are perfectly safe when they're serviced by a big truck tire shop with the right equipment. Another option would be to find 22.5" single piece wheels and tubeless tires. They will have approximately the same height as a 20" tube type tire. I'd figure on at least $1K for replacing the tires and wheels- - - -more if you have to buy new wheels. Brake work on these trucks can get spendy in a hurry, especially if you have to replace the hydrovac power booster. Some parts stores can send your brake shoes out for relining, or doing it yourself isn't terribly difficult if you can find the right width and thickness of lining material. NAPA stores can usually order wheel cylinders if they can match up casting numbers.
The GMC V6 is a rugged engine, but it loves gas. The 401 in my 1961 American LaFrance pumper truck just might get replaced with a 454 V8, but right now I'm hoping to be able to stay with the V6. Good luck!
Jerry
