First, as several here have noted, don't confuse the dangerous 2-piece split rims with the 3-piece lock-ring rims. Both my 52 'bolts have the latter. Also, in researching tire sizes, I found that it is almost impossible to find a 19.5" tire with the correct outside diameter to work and look correct on a 'bolt. Virtually all of the available tires are lower-profile radials - look at the 3500HD trucks on the road and you'll see that the tires all look too small for the truck. In addition, the smaller diameter tires will only worsen the problem of high engine RPMs at high speed.
Finally, and not to discount the dangers in mounting ANY tire on ANY rim, it is certainly possible to safely mount a tire on a 3-piece rim yourself. I have seen idiots seriously hurt themselves with conventional one-piece rims and modern tires; frankly it's evolution in action as far as I'm concerned. Also, if you don't feel comfortable doing it yourself, don't.
Having said that, the things to be careful of are 1) be sure the rims are in good shape and not excessively rusted. In particular, be sure the lock ring and mating notch have sharp corners and aren't rounded. 2) Wrap and secure heavy chains around the tire and rim, passing them through the rim holes. The chains ends should be bolted or locked to each other. 3) Use an air chuck that clamps onto the valve stem. Do not have your body near or over the tire when inflating - NOTE that this also applies to seating the bead on a conventional one-piece rim!
