Originally Posted by mtflat

What you said will probably be misunderstood. Late 50 and up have smooth bed sides inside and out. 42 thru mid 50 F1's have a style similar to the 48-52 F2/3's with a raised panel on the bedside. This is a different issue entirely.

I believe what you're describing is the line where the bedside meets the floor.
F1 boxes measure 49" wide inside. F2/3 boxes measure 54" wide inside.

So yes, the bump at the wheelwell allows for an axle width the same as the F1. They just shifted the box side outward 2.5" each side for a total of 5" difference. Basically the wheel well is "tubbed" into the box.

Also, the F2/3 fenders were adjusted the same amount and are 2.5" narrower than F1's.