...somewhere along between 53-57 I think the chassis kicks up more around the rear wheel area. I'm not positive on this but you ask the difference and I THINK it "kicked up" around '54.
....maybe I misunderstood your question?? Good luck on your decision making.
The frames are very different. The '55 2nd has straight rails, constant 34", no taper like the 1/2 ton ADs. The kickup at the rear axle is present, but not as pronounced as the '54s.
