This is the link to that unit http://www.speedwaymotors.com/p/5852,297_Mustang-II-Power-Steering-Flow-Valves.html
Others probably sell it or a similar unit.

Heidts has an adjustable unit http://www.heidts.com/heip27.htm#power
Scroll down the the page a bit.

1948 Chev pickup.