nice work on the frame jig !
i made a similar setup when i designed and built the chassis for my '33 dodge "humpback" delivery.
to make the jig moveable, i used a set of 7" casters with solid tires i had laying around.
if a person is considering moving something like this around on a lumpy shop floor, or outside [like to put on a truck or trailer], the bigger the casters are, the better.