These old trucks can be deceiving depending where you look and measure. I tend to measure the boards usually as it's a long expanse that's connected to the front and back fenders, and usually where your eye is visually drawn when looking at stance. That said, in a 60 year old truck things don't always line up as they should and sheet metal isn't always straight anymore (or wasn't to start with).

I try to make sure there's nothing else going on with the box and cab mounting/alignment before I dive into changing the suspension.

1946 GMC Pickup - S-10 Frame, 455 Buick, TH400, original patina.

My 46 GMC on Photobucket