I've used the Saginaw overdrive in a '51 GMC pickup, and loved it. Adapting it to a 4-speed Sag tranny is fairly simple - see the tech article at
Chevytrucks for details. What this article doesn't mention is that you'll need to relocate the shifter to clear the solenoid. Patrick's Saginaw/Hurst shifter makes this possible, but the mounting bracket has to be modified and the reverse gear rod and lever have to be flipped to clear the rest of the linkage.
Finding a Saginaw OD unit can be difficult, they were only used from '65-'72 when GM started using 5-speeds. The earlier Borg-Warner overdrives use a lot of the same parts, but the two won't interchange.
A true overdrive is great fun. It provides clutchless shifting (if you're in no hurry) because it contains it's own freewheel clutch. And the free-wheeling is a cool feature in itself, but you have to lock it out if you drive a lot of grades.
I'd still be using the OD except for the fact that parts are getting hard to find.
