Hey Goose, I looked into this a while back and although I don't have my laptop, I will post some good links.
IMHO there are two things to look at:
All you really need is a good stock donor axle that you can carefully remove the drum brake backing plates and carefully weld on the tabs to mount the econoline disk backing plates. I'll post the link and it will all make sense to you.
The other thing to look into is back a few years ago, replacement brakes were offered as a complete, from the backing plates to the drum or in this case disk, with calipers, pads, hardware, like completely assembled on a piece of cardboard with all the parts assembled and shrink wrapped. I haven't seen it in years, but back when I lived in Cleveland, it was a popular deal as the backing plates rotted out, the little keepers pulled through and then the whole mess of parts followed the rotating group
We used to call it "making rear brake soup", as all the parts were in there 
Anyhow, if you could find that replacement brake offering, and then you see the little piece of mount that was required, I think you'd have a winner.
Lastly: don't overlook the master cylinder. I would try the stocker on your truck but be prepared to replace it with one off of the E350. Don't let anybody tell you that the piston sizing and proportionality doesn't matter. I do hydraulic stuff for a living and having the "wrong" master cylinder can and will create too much bias in one or both directions.
Good luck: I think rear disks are an awesome upgrade idea
IMHO there are two things to look at:
All you really need is a good stock donor axle that you can carefully remove the drum brake backing plates and carefully weld on the tabs to mount the econoline disk backing plates. I'll post the link and it will all make sense to you.
The other thing to look into is back a few years ago, replacement brakes were offered as a complete, from the backing plates to the drum or in this case disk, with calipers, pads, hardware, like completely assembled on a piece of cardboard with all the parts assembled and shrink wrapped. I haven't seen it in years, but back when I lived in Cleveland, it was a popular deal as the backing plates rotted out, the little keepers pulled through and then the whole mess of parts followed the rotating group
We used to call it "making rear brake soup", as all the parts were in there Anyhow, if you could find that replacement brake offering, and then you see the little piece of mount that was required, I think you'd have a winner.Lastly: don't overlook the master cylinder. I would try the stocker on your truck but be prepared to replace it with one off of the E350. Don't let anybody tell you that the piston sizing and proportionality doesn't matter. I do hydraulic stuff for a living and having the "wrong" master cylinder can and will create too much bias in one or both directions.
Good luck: I think rear disks are an awesome upgrade idea
