I just ordered my Gear Vendors Overdrive!

I had one in my '70 Charger for about 18 months. The unit worked fine but the electronics were faulty. I liked having that extra "gear". Although the .78 ratio was not that great compared to a 518 (.69) or a manual trans, it does help a LOT for reducing driver fatigue.
My GV was a first generation unit. It had no provisions to mount the floor shifter. It had a bolt on yoke requiring a special slip-spline drive shaft. The new ones are much better.
Good luck.