The Ram 2500 is a cab and a separate bed on the frame (not counting the front sheet metal that has a mount on each side near the radiator for radiator and font sheet metal support), but the Town Wagon is a 1 piece unit ( not counting the front sheet metal which has a mount on each side near the radiator for support). I suspect the Ram cab has 4 or 6 mounts under the cab (depending on the cab) and 6 (or 8 depending on the bed length) mounting points under the bed, but was the bed bolted directly to the frame without rubber between? (Haven't been under enough newer Rams to notice any rubber between the bed and the frame).
The Town Wagon, since its a one piece unit (not counting the front sheet metal) probably has 6 (or maybe 8) body mounts under the body unit (in addition to the front sheet metal) which will need some sort of rubber between the entire body and the frame. It will also probably have to stay level as it sits on the frame or you could have issues with door open and close fit and window cracking. I would probably use the Ram style cab mounts under as much of the Town Wagon sheet metal as you can and still keep it as level as possible doing that. You may end up adding extra rubber, or steel shims (large diameter washers work) added as pairs (at each side at the same forward or rearward positions) so the wagon body doesn't bind up as the mounting bolts are tightened. If you are using bolts with nuts, I would recommend using nylon lock nuts instead of regular nuts with lock washers. Be sure whatever location you are placing the body mounts on the Town Wagon body's floor pan, that area is reinforced. If it isn't and the rubber is just against the sheet metal floor pan, over a pretty short time, it will crack and start splitting the floor sheet metal.
You are going to do a build thread with pictures of this build for us, aren't you?
This is exactly what I was thinking but you said it much better! You are pretty close on the body mount count. I might box the frame in or add a cage to alleviate the flex as much as possible. This to me seems like the best solution for the Frankenstein body mount situation. The frame already has added cross members from its previous life but I will have to add more for the rear 4 link eventually.
If I started a build thread it would be the slowest thread in the world and I wouldn't want to torture anyone.