A 79 Dodge full size van DOES have a frame. The steering would have to be re-worked though.
If you want to update with minimal work just pull the engine and drivetrain. A 48 Dodge should have a solid axle in front. Those are easy to maintain. While it won't ride the best on a bumpy road, it will be fine on smooth pavement, and as good as 70-80s 4x4 trucks (solid axles in front) all the time. The engine is the main problem as far as maintenance. The manual trans shouldn't be. With the old engines you could be stuck on the road if you have a problem. Say if a water pump goes out -- you might be stuck for 3-4 days while one is next day aired in, and that won't be cheap (hotel room or shipping!). It would be a lot cheaper to make motor and transmission mounts (or might be a kit!) on the existing frame and change the engine/trans/rear axle (you need to match ratio to the engine and trans used). That takes some work, but not nearly as much as modifying a frame to fit the old body.
That said, what you are asking about CAN be done. A friend has a 50s Studebaker on a Dakota chassis. He cut the floor and firewall out of the Dakota and Studebaker (which needed floors anyway) then trimmed the Dakota to match the Stude dimensions. A "little" welding later he had a good fit. The front clip took a little more work. He used the firewall from the Dakota so he could keep the AC unit and all. He's happy with the work, but it did take a while.