I did some pretty extensive research on high-geared GM rear ends when I was working on my wagon. At one point, my wagon had a 2.14 rear, .73 overdrive (NV3500 transmission) and the factory 350 motor. It would do 55 MPH at just under 1000 RPM. At the time, I was using a heavy, 4.3 V6 flywheel, which helped with low RPM torque, but yes--the engine pulled that speed/RPM just fine. In fact, I would often cruise at 45 MPH and ~800 RPM.
The highest ratio that is available for the 8.5" 10-bolt in your G20 is 2.41. The 7.5" 10-bolt (which is what I put into my wagon) was available with 2.29 and 2.14 ratios, but that might not be heavy duty enough for your van, depending on how much you haul. (Astro vans had that rear, though, so they're tougher than you think. I don't know if an Astro van would swap into your G20 or not. They have the same wheel bolt pattern, but the differential is off-center and it may be a bit narrower.)
The only other option is a fairly rare 8.75" GM rear, with 12 bolts on the ring gear and 10 bolts on the cover, that came in some cars in the late 70's and early 80's. There was a 2.28 gearset for it, but that gearset only came in Cadillacs from 1977-1979. You could weld spring mounts onto one of those Cadillac rears and bolt it into your van, as it has the same wheel bolt pattern (5x5"), but it would be narrower than your factory rear and the axle shafts and outer bearings would be smaller/lighter duty. If you wanted a rear that was roughly the same width and the same strength (same shafts and outer bearings) as the factory rear in your van, you would need to find one of these 8.75" rears from a late 70's/early 80's B-body station wagon and swap in the 2.28" gearset from a 77-79 Cadillac. I seriously considered doing just that with my wagon, before I fully embraced P&G with EOC and gave up on extremely high gearing.
I hope this helps! Let me know if you have other questions about high-geared GM rears. Unfortunately I know very little about Ford rear ends.
-Funkhoss