cjken - frame rails are standard US truck (Freightliner, etc) 33.5 outside from memory.
Ross - salvagetrucksauction.com might be a source.
Very slow progress on my rig, I'll post some picks when I get it in the sun.
I built a 4"x2" subframe, mounted to the chassis on 3 points, using Freightliner class 8 front spring bushings, about 4" long and 2.25" diameter, with a 20 mm bolt hole.
I took the subframe to Complete Bodies, just south of Atlanta and they're going to build me a standard box truck box directly on my subframe. I went this direction so I can have a nice new box, really well built, to my specs, with a good chassis mounting system. It will be fully waterproof, with a very robust exterior, really quite light weight, and an excellent starting point for fitting out.
14' 6" long, 8' wide, 85" headroom inside. There will also be an 36" attick over the cab for storage or a single bed.
The box will have a hardwood floor which we intend to varnish, should look good. The structure is 1" thick "Z channel", clad with white painted aluminum (or aluminium). Roof is also aluminum.
They will be cutting my window & door openings as well.
My intent is to fir out the framing using 1 5/8" (1.625") x 20 ga wall studs to give me almost 3" wall thickness, run all the wiring, etc before filling with either styrofoam, or maybe spray insulation.
Fresh water tanks, 2 x 30 gal, are mounted between the frame rails, behind the axle.
I'm planning a pretty large solar system, so 2 boxes mounted on rails also behind the axle to each hold 4 golf cart sized batteries. The boxes slide out on rails I built for maintenance, etc.
Will hopefully have the box soon, and then my wife ("trouble & strife") will have something to really get interested in. I'll put the box on the floor while fitting it out, then figure out how to lift it up onto the truck later. The bare box will be about 2,200 lbs, and kitted out, less than 3,500 I hope.
Bob