The AMT '70s Ford is a "Camper Special" though, 8 foot box but longer than standard wheelbase. That rule will not work with that particular truck.
OK, that does change things a bit for sure! A couple of things will be necessary to get the essential measurements: First, the wheelbase of the short box version, and second, the BBC (Bumper to Back of Cab) dimension. The latter should be fairly easy to find, as virtually all light commercial sales literature (beyond the smplest sales brochures) back in those days (and probably still do) will show that dimension, for prospective buyers of a cab & chassis version for mounting an aftermarket commercial body.
However, even without that, just having the overall length of the short bed pickup, along with the wheelbase, will give the essential information. In any event, doing a proper short box conversion will involve shortening the box and chassis forward of the rear axle, and with the camper special, there still could be some alteration of the rear overhang needed as well (but that would not be known without the two dimensions I mention in this paragraph. No matter what, it should not be an insurmountable task though.