The old school was of doing an overdrive was a Fairey overdrive. They haven't been made in a while, but you still may be able to find one. They were made for three speed trannies and I think they were a bolt-in unit, since they were made specifically for the FJ40.
Whoa. I just googled Fairey Overdrive, and it looks like someone is making them new for Land Rovers
. Might be worth giving them a call.
There's a whole thread on MUD about these units.
I've also heard of guys doing a Ford Ranger overdrive, but I don't know anything about it.