Welcome to the group.
As you already see you will get a lot of different ideas and suggestions on here. Some are workable and some just won't fly with you plans for the truck.
First I would take stock and decide how I want the truck to look and what plans I have for it. From what I see you want it to still look like grandpa's truck did but have some modern brakes and a more modern engine that might be able to do road trips better.
Engine wise the late 60s an up though the 70's 292 sixes still do duty in a lot of trucks that size. The other option is to swap to a V8 and maybe a 5 speed ovedrive transmission out of an 80 something truck.
Disk brakes on that axle. Right now I don't know of any setup that will bolt on. You might look into late 70's or though the 80's 1-1/2 and larger trucks to see what sizes had an I beam axle and disk brakes. For the later axle to work you would have to have the center of the spring bolt hole in each side of the axles be real close if not exact so the axle would swap or see if the king pins are the same or close enough that with some machine work the spindles would swap.
This link takes you to a stovebolt thread on swapping disk brakes on the bigger AD trucks and may help once you read through what he is saying a couple of times.
https://www.stovebolt.com/ubbthreads...25027&page=all
The thing is that most guys with that size truck either restore them back close to exactly as they were when new of the dealer's lots or do a lot of swapping around usually just keeping the cab and end up with a truck that looks nothing like it did from the start.