[fleetsidelarry]

slammed, on my long bed fleetside there are 3 cross sills and 2 frame cross-braces after the rear axle. one of the cross sills sits midway between the 2 frame braces. I think most guys put the tank between the 2 braces, if so you have 27" of width and 16½" front-to-back (the outside-outside measurments in the diagram above aren't really helpful because the frame rail width changes and the lower lip is a wider section than the top). unless you leave out the cross sill, you can't install any higher than the top of the frame. the frame also tapers in height at this section but is about 4½", meaning of course that any tank depth beyond that would hang below. cross sills are 2¼" tall so you could gain a couple of inches by leaving it out, but I don't think I would.

I would think the shortbed is the same behind the axle, but I'm not certain.

There was a thread and some links on the subject recently you might try to find.