I'm glad to of found this thread, lots of great work and ideas!
I'm new here but I have converted a few E350 vans to 4x4, I think the first one I did was in the late 90's. I was into regular vans until I found a 89 E350 Ambulance with the 7.3 back in 2003, I converted it to 4x4, did some interior work, then drove it till it finally rusted out 11 years later, best work rig I ever had!
With so many accumulated parts over the years, plus now a extra parts Ambulance with a good 7.3, I just purchased a 90 E350 Ambulance with low miles(for a ambulance 128,000) that was used as a Dive Rescue vehicle since 96, the body has very little rust.
So I will be putting a Dana 50 TTB up front, which I already had two in case I found another Ambulance, plus other things as I go. I will also be converting the old aluminum body rear box to a trailer to tow behind the new(for me) ambulance.
Thanks for all the info. look forward to reading more!