[SS Tim]

Adam, Ran the numbers and it looks like a 157" 1 ton frame might be the best choice, especially with a big block. The '72 Dodge swb crew cabs sat on a 145" but your cab looks a little longer. Take a look at the drawing and the numbers should make sense. In the end with a cab in the stock location and a short bed (6.5) centered on the rear axle you would have 42" to play with. Maybe move the axle depending on where the cab finishes out or bob a long box at both ends (front to fit and rear down to short box over hang). As a note the Scotbuilt type four doors added 42" with full size doors.

157"wb +33.25" (bumper /front axle CL) =190.25"
190.25"-106.25" (BBC, bumper to back of cab) =84"
84"-42" (cab to center of rear axle) =42" over stock available for extended cab.