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

You can buy the 18-spline heavy duty OD conversion gear set from Jamie Passon and add it to your existing 4 speed. You will have a heavier duty transmission and a better OD ratio than the GV. I had a GV unit on a 727 in a 71 Challenger. If I had that to do over again, I would have used a 518 and done the mods to the transmission that @1WildRT came up with years ago to just bolt it right in place.