Has nothing to do with Quigley. They do not set such ratings.
Depends upon gearing first and foremost. Curb weight second. Being a conversion van it will weight more than stock, so this detracts from your payload capacity and tow capacity.
The E350 should have the same GCVWR as the F350 of similar vintage.
3:55 geats = 16,000 max GCVWR
4:10 gears = 20,000 max GCVWR
So take one of those numbers depending upon your gearing, and subtract your curb weight. Say your van weighs 10,000lbs and you have 4:10 gears, then you have a 10,000lbs tow capacity.
With that said, if you need to haul anything with more than a 500lb tongue weight (5,000lb tow) you will need a weight distribution hitch to so it safely.
The only way a E series can tow its max rated tow capacity is with a WD hitch.