The shocks make it 1950 or later, like Dan said. The higher hump over the rear axle make it 1953 or earlier. In 1954, the flat bed sides and deeper floor required less of a hump over the rear axle.
So from those two things, it's a 1950, 1951, 1952 or 1953 frame. Can't get any closer than that.
The tire size doesn't define what the wheel hubs are, as there are lots of rim sizes that will fit on the 6 lug pattern.
