Measuring his 1953 C body V8 frame, a friend found .085, which is consistent with a lightly rusted 13 gauge (.089). Frames were generally thicker for Canadian and export cars, usually 11 gauge. The 6-cylinder cars were thought to have 14 gauge frames.
Besides six and V8 differences, the C/K models had increases in the frame thicknesses at a few intervals during their production years of 1953-1964.
Here are the specs from the '59-'64 Shop Manual... if anyone wants to copy them.
Year Body Frame Thickness
59S W, F .0747"-14GA
59V W, F .0897"-13GA
59S-59V D .097"-13GA
59S-59V J .1196"-11GA
59S-59V Y .1196"-11GA
59S-59V C* .0897"-13GA
60S W, F .0747"-14GA
60V W, F .0897"-13GA
60S-60V D,P .0897"-13GA
60S-60V J, L .1196"-11GA
60S-60V Y .1196"-11GA
60V C* .0897"-13GA
61S W, F .0747"-14GA
61V W, F .0897"-13GA
61S-61V D,P .0897"-13GA
61S-61V J, L .1196"-11GA
61S-61V Y .1196"-11GA
61V C* .0897"-13GA
62S F .0747"-14GA
62S-P, 62V-F P .0897"-13GA
62S-62V K, J, Y, .1196"-11GA
Taxi-Police .1196"-11GA
63S .0747"-14GA
63S-P, 63V-P .1196"-11GA
63S-63V K,J,Y,L .1196"-11GA
Taxi-Police .1196"-11GA
63V F .0897"-13GA
64S F .0747"-14GA
64S-64V P .1196"-11GA
64S-64V K,J,Y,L .1196"-11GA
Taxi-Police .1196"-11GA
64V F .0897"-13GA