Moving the hinge to the ground was Kevin Olson's suggestion. Since they were full forks, I figured "Why not"? It will look like a hand-vise for the Jolly Green Giant, but it ought to be sturdy.
After checking load ratings for Thomson ball-screw shafting vs acme, I ordered 1-1/2" acme & three nuts. At <$90 from Mcmaster, I'm fairly pleased to have that be the only purchase so far, and that's enough shafting to do two more vises, maybe.
On the scrounge for something both hold two nuts, in time, to prevent them locking. I have a 4" ball hitch, but it's justs looks too big. Hunting a 3" & several buddies looking too. Still need something fairly heavy wall for the hinge barrel & settle on an initial mount design....
If only work would stop interfering with my play time.