Just my $.o2, keep the 3.90s and get an overdrive. They're not too hard to find, and fairly aeasy to work on, too. Parts can be an issue, especially electrics. But as noted, the overdrive unit itself is pretty much the same, regardless of what trans is in front of it. One suggestion I would make when looking for one, though, is don't look for one out of a truck, look for one out of a car. The reason is the car version (virtually identical other than serial numbers) is much more common. Expect to pay somewhare around $100-$150 for one. You may have to use the levers/side cover from the original trans to make it work.
