[Jim A]

When I got my '60 1500 in 1970, it had 17.5" stock solid rims.
GMC apparently didn't know which trucks had which and offered me free replacements for 16" split rims and tires.
The replacement campaign went on for several years and I am not sure when or if it stopped.
Since it was considered a severe safety hazard, I wonder if the offer might still be valid with enough prodding. Worth a try.

FWTW, nice, but expensive, 17.5 tires are still available