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

The GV is a lot easier to install but also has an overdrive ratio that isn't that great.
I had a GV years ago. I had 4.10 gears with it at first but changed to 3.91s later. The GV ratio of .78 did make a difference in how the car felt on the highway but the Tremec 5 speed I have now has a .64 OD. Now that really made a difference. That is a 36% rpm reduction compared to the 22% that the GV unit offers.

If you ordered the Tremec, you could recoup some of the cost by selling your 4 speed stuff.

Check out my thread on my Tremec installation. It is very thorough. I even included a list of what everything cost me.

Tremec 5 speed conversion in a 1970 Charger