The Ford ambulance swap should be a bolt-in or, at the least, require minimum fab work. Not all will fit of course. If you can find a bone yard that buys a lot of Ford ambulances you might get lucky enough to measure, pick and choose. The ones that will work are the ones made similar to the dually pick-ups. The inside rear tires will track closely to the front tires and the outside rears just stick out there.
If you go with some other manufacturers set-up your fab time and hassle will be aligning the driveshaft as different manufactures offset the engine/trans differently.
As an aside, there are vendors that sell dually conversion kits (including fenders) for E350 vans that use the existing 8-lug differential.
Edit: The 7.3 vans came with a smaller turbo than the pick-ups (and a slightly lower HP/torque rating). If you can make it fit, the larger turbo will help bring the power up to the pick-up level. Finding room under the doghouse will be a challenge, Ford couldn't do it.