I have a 1977 F150 longbed, 351M, w/ C6 and am interested in upgrading the tranny to an overdrive. either factory or aftermarket attachment. I have never tried this and was wondering if it was possible or even feasible?

 

You could check with the folks at Baumannengineering.com. Their site says the overdrive version of the C6 is the E4OD where Ford attached an overdrive to the front of the C6. Not sure it would be easy, but they might know or have a kit.

There also are companies that make two speed gearboxes that go in after the tranny to give you over drive. Not cheap. Maybe $2000. Saw in a motorhome magazine, but can't remember anything about it but that maybe they are called "splitters"? Not much help here, but maybe someone else will post.

 

Those company names are Gear Vendors and US Gear. The installed price is more like $3000-$3500 with driveshaft length modifications. Type '.com' after thier names to get thier websites, which are very informative.

The E4OD is an electronic transmission so you would need all the wiring and computer to go with it. In your older truck, an AOD would be a better choice, but I'm not sure if they come with the 351M bellhousing bolt pattern or not. I'm sure someone here will know that though.