denny,
you can use most (non split) rims from any 3/4 ton chevy and they should fit. you will have to use some kind of spacer between the drum and the back of the rim as the wider rims will contact the tie rod ends. just doesn't want to drive the same when you can't steer (lol). the spacer will reduce the amount of threads the lug nut will spin on to, but most of these trucks don't see the same level of work they did in their hayday.
i am curious about the 15" rims you are referencing. i have to assume that 15" inch rims are what you want to mount and not what was on the truck originally.
my 50 3/4 ton has the original rims which are 17" split (lock ring actually). i am running 6 plys on the front and 8's, mud and snow, on the back. once i had gone through the steering (tie rod ends, drag link ends, king pins, bearings, etc. it runs straight down the road barring the occasional pavement irregularity.
if you want to go for as much stock appearance as possible, you can drill and rivet or even weld tabs to hold the original hub caps on modern rims.
