The rear axle is the simple one, width is not as big an issue because a flat bed is usually 8' or nearly that wide, and most axles with tires installed stay under 8', some narrower.
Here's a Rockwell/Meritor 12,000 lb axle I used on another truck. It had lots of gear choices, disc brakes, and easily available 22.5" wheels. Came from a 2005 Freightliner, went into a 61 F350.
http://rides.webshots.com/photo/2231135140080251109IqQZWvI' like to keep an AD truck as narrow as possible, because they are narrow to start with and look better to me that way. The rear axle I used from a P30 is just a couple inches over 7' with tires, and couldn't be any narrower or tires would rub the springs.
John's truck we're using a Dana 80 with 6 lug Budd wheels and disc brakes, forget what it came from, but it is also as narrow as can be, actually had to find narrower wheels than what we originally had so they would clear the springs. It's even a bolt in swap, spring perches are in the right place already.
Lots of rear axles to choose from....
The imported cab over trucks look tempting, and I was looking at some for that F350 project i had, but kept coming back to gearing, and they all had way to slow gears. I needed 3.54 or 3.73 and they were in the 4 and 5 range.
Wheels should not be a problem with them, 22.5" and 19.5" are available with the imported 6 lug stud centered bolt pattern, no need to adapt to something else.
The Dodge and Ford tubular front axles, or any driven front minus the differential for that matter, looked tempting to me too.. But think about the height of the truck, no room between springs and frame to mount the axle and have any travel left. And if you mount the axle under the springs it lifts the front of the truck about 5" over stock.
Steering arms and linkage is not an issue if you are also swapping steering gear boxes, lots of combinations there to make things work.
Here's my 3500HD axle with IH Scout II box and drag-link that goes left to right.
http://rides.webshots.com/photo/1484207681080251109ppdzRrNext idea is to use an Isuzu cabover box and P30 steering arms and linkage with front to back drag-link
http://rides.webshots.com/photo/2436693320080251109WnhCjyhttp://rides.webshots.com/photo/2024492560080251109VNDanJDon't think you'll find any spindles to easily swap and gain disc brakes on the original axle, I've already looked pretty hard. By the time they went to disc brakes on Chevy trucks they had forgotten about I-beam axles. It's was not until the 3500HD and P30's in the late 80's came along that they found an I-beam axle and disc brakes again. those spindles and brake parts are your best chance at adapting, but in my opinion it's better to narrow the whole beefy axle and not fabricate and adapt to the comparatively small original axle.
There is no "easy answer"
Grigg
