Anyone using a Gear Venders O/D unit?

I'm running one in a 68 Dart with 4:10 gear 904 trans and a 28" tall tire. I bought mine used and installed it myself. The unit is suppose to fit the tunnel without modification I found that I had to drop the trans mount down 1/2 to 3/4 of an inch to make it fit without tunnel mods. The GV directions say you may only need to drop the trans .100 So that is the only real compliant I have. In my car it drops the rpms down from 3000 to about 2500 +/- cruising on the freeway. Can't really say what kind of milage increases it offers as this car is more of a weekend driver and mileage is not really an issue. The real benefit for me was being able to keep up with traffic here in So. Cal. I can now cruise in the fast lane doing 75-80 at about 3000 rpm instead of the truck lane running 65-70 at 3000 rpm.