Thinking about a Gear Vendors OD. Can Anybody Give Some Guidance?

I drove my 69 340 4 speed Barracuda to a car show on Saturday. It was about 90 miles each way. Every time I drive my car on the highway, I think about how nice an overdrive would be. I have 205/70/14 tires (25.4” tall) and an 833 with 1:1 final ratio. At 65 miles per hour, I am at about 3100 RPM. I guess that isn’t the end of the world, but after a while, it is a little nerve racking to me. It would be SO much nicer to be cruising at 2200 (or so) instead. I found some older threads about Gear vendor installations, but nothing current. I REFUSE to buy one of Passon’s 5 speed transmissions, and I really don’t want to go the Silversport 5 speed route. That looks like way too much work. The Gear Vendors installation LOOKS easier on paper. Any thoughts?