Apparently you asked a hard question. I was interested in responses, but none have emerged.
For what it is worth, my 62 - cross members appear to be same as pickup bed in size and shape. placement and number likely not consistent.
But it makes sense the the standoff height from the frame is the same for subs and pickups based on the alignment of the doors and front fenders. Also makes sense that the length is consistent. Sub fuel tank plays a factor in location.


~ HB
1966 Chevrolet K-10 | Ghost: formerly Flappy Fenders | In the Stovebolt Gallery
1962 Chevrolet C10
1962 Suburban