At one time there were people overhauling the governor and the solenoids. Do a Google search and try to track someone down. There is another post running on an O/D Saginaw '69 installation that I posted some pictures and links. One of which is making a control box to make it a true six speed. But why are you not using the kickdown switch? They are available.
http://67-72chevytrucks.com/vboard/s...d.php?t=549797