I vote for the 3 speed od as well. I recently put one in my 1966 GMC with 305E v6 and am very happy with it. The three speeds came in two versions, the early ones prior to 1966 were unsynchronized first gear and had 4 bolt side covers. The 1966 to late 60's had a synchro first and a seven bolt side cover. I have the later model tranny. Yes, it has an electric solenoid that activates the OD. Here are the original manuals on the tranny if you would like to read up on them. They come up on Craigslist and E bay fairly often. The only problem is they are expensive to ship. You should have the seller remove the solenoid and governor switch if you decide to ship. UPS broke the governor on mine in transit and wouldn't pay for it even though it was insured. These two items stick out from the tranny when attached and are prone to breakage in shipment. Here's the link. Let us know if you need more info.
Regards,
Steve
New Orleans

http://speedprint.com/deves50/overdriveindex.php