Spring Air #704 are available for ~ $127 US delivered, they include everything needed, www.retracts.com. Robart also has a set of spring down, 85 degree retracts, don't remember the price.
As to the Williams Brothers wheels, if you're going for scale position, 4" robarts are about the largest you can stuff in, and scales are 4.7"-4.8". As I recall the Williams Brothers are in the range of 4.5".
Before you commit to them, make a mockup of the wheel and see if they will fit in the well. Just remember that the wheel doesn't come straight up, but because of the forward tilt of the mechanism moves both FORWARD and UP.
So far, it's been many hours with 4" Robarts trying to get the wheel to come out of the well. Just remember to try mockup before you commit.
Bob Q.

Scalecraft'

Steve,
Many thanks, the pics are just what I needed. I have to get a contour gage.
Bob Q.