1. Who is going to do the work? You or a shop? I see you are an engineer and gear head. That's good. Many of us here are Gear heads and engineers, shop instructors, experienced builders, DIY specialists and fabricators.
2. An early, but recently rebuilt 350, is perfectly good. A 350TH is good, a 700r4 would supply overdrive for "interstate trips" and a easy swap. Don't need a bigger motor.
3. Everything you want is available in kits, donors, rebuilt/new FLAPS and aftermarket Classic Truck vendors.
4. I would start with the front axle: disc brakes and PS. Tapered roller bearings or a whole new/donor axle that would have it all.
5. You say "everything else is original and ask for rear end suggestions. You can't possible have a 350 motor with an original 42 rear....could you???????? Could be, the axle is full float and strong. But 4.55 ratio!!!!!!!!!!!!!!!!
6. You are in a "IFFY" area, I always recommend keeping the frame and adding axles and restomod components instead of an aftermarket preloaded frame or donor, but I don't know how hefty the 3/4 frame. My guess is it is overbuilt for the era and will be OK. The GM heritage site will give dimensions and thickness of frame channel. So you can check that and I will also. Another factor I would have to consider is what all was added, modified, cut off, compromised of the 42 frame by PO's.
7. I would keep the leaf spring design front and back. It's a truck and will not ride like a car. The leaf spring system allows for much, much easier axle swaps. I, myself don't favor Mustang'ish, Fat Man'ish, clips and etc.
8. Placing an aftermarket preloaded frame under the body or some S10 BS will seem like cool and easy do. But will have a whole new set of issues.
9. It's hard to advise without see the whole undercarriage and all the surprises you forgot to tell us or haven't seen.
10. There will be members here who have done this stuff to a 40's truck. I have only done it to 55.2-59 trucks. Fairly similar though. Take all of the Blog Sites and How-To info with a grain of salt as you search for info.
11. The production of civilian trucks during a WAR year would be very low. Your truck is pretty rare and in great condition.
12. Your Vin may disclose which of the two available 3/4 models you have. The numbers on your rear axle and other parts can be of help if keeping any of it. Those components you are swapping will be sellable.
VERY IMPORTANT
Make a list or a drawing of all dimensions.
1. WMS front and rear
2. Frame width
3. Ride height
4. Wheel base
5. Radiator to fire wall
6. Take many measurements and pics of "AS IS" truck chassis/motor/trans/mounts/axles/steering/gas tank....for future ref. Some of the measurements/specs will be in the GM Heritage Documents online.
1942 3/4 T has ladder frame with 5 Structural Crossmembers, Frame build from 42K psi cold rolled steel. C Channel is 5 27/32 tall, legs are 2 1/4, thickness 3/16. Strong enough.
Due to being a 3/4 T, the suspension and axles are strong. Stiffer ride than 1/2T
