Well. The 7.3 came stock in the E350 econoline and can be done with all stock ford parts. The Cummins, mating transmission and transfercase would be a very, very difficult to stuff into an econoline due to the length. Everything would have to be custom made. The tunnel just isn't that long, the 7.3 is really tight... the gasser is tight but not that bad, I can still get to the plugs and manifold bolts.
The issue is CARB who made it very clean that I can't do a engine swap on this van due to its published GVWR. It's listed as 8100lbs and the cut off for medium heavy duty is 8300lbs.
Basically I can swap any engine I want so long as it came from a vehicle under 8300... which there isn't a diesel made that falls into that range.
However, I can re register the engine as a 91 F150 and do pretty much anything I want to the engine so long as the parts as CARB certified... which there is all sorts of fun go fast parts that will still pass the sniffer and inspection. AFR heads, headers, intakes, cams, etc. I can build a serious brute with the stuff available.