According to my book which is Chevrolet not GMC:

Model 3105...6.70-15/4PR
Models 3106-16...6.70-15/6PR
These models could be ordered with optional 16" or 17.5" wheels.

All other models 3155, 3805, 3855, and 3156-66 came with 16", 17.5" even 19.5" wheels.
Bruce


Bruce
Old Toy: Ol' Betsy stock 59 Apache SB Stepside (Gone but not Forgotten)
New Toy: 1962 Impala SS Coupe
The Driver: 1990 Toyota truck