If you are trying to maintain the stock look, the 3 speed overdrive is the way to go. I have a 58 Suburban that I did the swap in about 6 months ago. It was an excellent change.
As far as I know the three speed and rear end in the truck were stock/original when I got it. In third gear I was running 3,000 RPMs at 55mph. Like you, I thought it was too high for comfortable driving. It wasn't that the truck wasn't capable. It could run like that all day long. The problem was I couldn't hear myself think when driving and it drank the gas.
After the swap, it runs 2100rpms at 55 and will run 70mph without any trouble. The mileage is better as well.
I bought my tranny on eBay. It came from a 67 chevy C10, so it is a Saginaw with the synchro 1st gear. It is rumored to be a stronger transmission and more desirable. I paid $535 for mine. That may seem like a lot, but it is 100% complete. It came with the complete wiring harness including the relay, kickdown switch, governor and solenoid. All I had to do was acquire a new yoke for the front of the drive shaft. The later model transmission had a different spline count on it's output shaft. It once I got the yoke changed, it was a simple swap.
Anyway, in my opinion is definitely the way to go. You get to retain the 3 on the tree tranny but also get the benefits of an overdrive. It makes for a whole different driving experience.
If you decide to go that way, shoot me a PM. I've got a few lessons learned that may help you.
