I think I found 7.50 or 8.00 x 20 Highway treads from Allen Tire ( Mid-Atlantic region) about three years ago, when I was looking for tires for my 55.1 4400.
You can definitely get "antique tires" from Coker or Universal, in B.F. Goodrich, Firestone, U.S. Royal, as well as Denman, but they'er not cheap, and probably not feasible for the "working truck".
The 20" rubber is getting a little hard to find because the truck makers have been getting away from 20" rims over the last 10-15 years, or going tubless & radial... most tire stores don't want to bother stocking such a slow-moving item... the two big local dealers in truck tires didn't even want to talk to me about 20" tires... couldn't wait to get me off the phone...
But there are still many, many trucks running around the US on 20" bias-ply tires and multi-piece rims... will have to rummage through my notes and see what I found...
Here's one possible source:
http://www.nebraskatire.com/truck/overstocks.htm And here's the "truck tire pages" from Universal:
http://www.universaltire.com/cart.php?target=category&category_id=343 Another:
http://www.tirexusa.com/simex/trbias.html
