Trich,
Yes, 3E indicates a 3600 truck. According to the GMheritage center docs there was a dual wheel option for 1957 3600 trucks. To verify that yours was equipped from the factory with duals, check the GVW rating. A GVW of 6900 lbs. should indicate dual wheels. Also, you may want to check the wheelbase: 3600s had a 123.25" wheelbase, while the 3800s had a 135.0" wheelbase.
I don't know a whole lot about these trucks, but I do own a 1957 3600, a 1956 3800, and a 1959 3800.
Oldbolter
1959 Apache 3800 dually flat bed/dump with 261
1956 3800 dually flat bed/dump with 235
1957 3600 flat bed, 350 engine, 700R4, PS, disc brakes, 14 bolt rear with 4.11 gears
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