Overdrive options for 3OTT?

[Oregoon]

Hey all, been a while but I'm still lurking around and still loving my '74 GMC 1500--driving it daily, getting around 18 mpg mixed city/highway out of my desmogged 250, and haven't even hit the truck's first 100K yet!

Anyway, I'm going to put a clutch in it before long and that's got me thinking about overdrive options. As I said, my truck is a relatively low-miled survivor, and in the name of low-rpms and longetivity, I'm wondering what I can do for an overdrive.

I'm a slow-poke and I like backroads, but even there, an OD would be a help.

I'm also an avowed fan of the 3OTT and so I'm wondering if anyone has any idea what I can do to enjoy both my column-shift and an overdrive unit?

Thanks as ever!