Yes, the base tires were 6.70-15, 4 ply.
There are RPOs listed for the '57 3100 and 3200 models, for 6.70-15, 6 ply, and for 6.50-16 and 7-17.5, in the Chevy specs as documented on page 208 of the
Vehicle Information Kit, from the GM Heritage Center Archives website.
Doug