It does look like a home made tandem setup to me.
The 22.5" tubeless wheels may be found on buses, but your best bet is to look for the wheels, not the truck. I have found several just laying around in junk yards, with no truck attached.
Even if they are on a truck they may not be original to the truck, same as you are planning to do. The vast majority of the 56-59 1.5 ton chevys I have seen still had the 20" wheels any how, seems that the tubeless were not accepted when they first came out, and folks ordered trucks with the tube type even though tubeless was standard.
You just have to start looking, they will pop up here and there.
Grigg
