the physical swap part is harder than on a truck or car, i think.
and then it depends on what diesel you swap in. a diesel has less to operate it then a gas engine, just fuel really. but only if its old. i don't know what engine would go any in with out a swap of quite a few things. even a 6.9 from 1983 would likely have too many "things" that have to go with it. but i bet it would bolt in.
if you look on ebay, very often there are E-series cab and chassis that were former ambulances. they ususlly go pretty cheap usually less than a PSD core charge, and they usually are running and ready to go. with one of these, you have all the lines, wires, brackets, exhaust, etc. from there its pretty much RnR, unless there is some body differnce I'm not aware of...
I have a '75 Econoline and it has gone from a 351 Windsor to a 460 and now a '99 powerstroke turbo diesel. I used a '99 ambulance parts vehicle and moved everything over as listed in my signature below. I used '91 motor mounts for a 7.3 diesel before turbo since the block is the same and it was last year for 75 1/2 - 91 body style/frame. Cut out firewall, engine tunnel and cab floor and replaced with same from '99 parts van and used all the '99 wiring including dash plus feul tank and lines. Works like a charm and have hauled 8500 lb trailer cross country several times and down to Florida and back to RI once.