If your truck is a 3604, it would not have been a duallie from the factory. The dual wheel option on 3/4 ton was only available on Stake, Platform, Chassis-Cab, and Flat Face/Cowl and Windsheild trucks.
Your truck would have had 8-19.5 6 ply. The 16.5 wheels probably came from a late '60s-early '80s truck.
