If commonly available parts are the big concern, I think that is still achievable with the original frame and only drive-train and or axle swaps.
That's the approach I have taken, the original parts on my truck can be counted on one hand.. Frame, Cab/sheet metal, steering wheel, pedals + brake lever, and the rear axle spring saddles. Everything else is newer, most importantly the front and rear axle, engine and transmission, and the steering and brake systems. I can get all the common replaceable mechanical parts at a regular auto parts store with minimal if any delay. The remaining original parts I may not find so easily are all related to the cab, I see no problems there.
If you still like this approach you would need to consider the original springs and hardware. The springs I used were custom made, although I only really needed the fronts custom to make the chosen axle work. I used the original style spring pins and bushings which are still available from most spring shops, I had no problems getting new ones and they did not come from one of the antique parts catalogs.
One downside you may bring up: is the parts are all available, but only if you remember what they were originally from, that's where good notes come in. If you swap a whole rolling chassis the note taking is much simpler..
Back to your original frame, if you have a short wheelbase (137") 1.5 ton, a model 4100, then the frame is almost identical to a 1 ton frame. Only difference I can find is the way the front bumper mounts, otherwise same thickness and channel size. Is this light enough for you? If you want lighter then you will probably end up with smaller wheels and tires, which may look out of place in the big truck fenders, at which point you would want small truck front sheet metal. If you go so far as to swap to a smaller frame and front sheet metal you may as well have started out with a smaller truck in the beginning.
If you have the long wheelbase (161") 1.5 ton, model 4400, then the frame is the same as under a 2 ton of the same wheelbase, a good bit heavier than the 1 ton.
What will you do with the truck?
Grigg
