They're nicnnamed "Frito" 19.5's because they were commonly used on the aluminum-bodied step vans that Frito runs. They're the same 8x6.5 pattern that your existing 16" wheels are.
The problem you'll find is getting a tire "short" enough to run a 19.5 on a static dropped dually. The smallest tire is a 225/70/19.5 which is about 32" tall, a hair larger than a 235/85/16. So good luck with that.
Here they are on my old '98 C3500 wrecker, the tires shown are 245/70/19.5's which are about 33" tall. You've seen this truck already...
Don't confuse those wheels with the ones used on 3500HD's, which are 10 lug, not 8.
Richard