A500 or 727 with GV?

The A500 is based on a 904 with the OD unit on the back end in place of the tailshaft.

An A518 is based on a 727 with the OD unit on the back end in place of the tailshaft.

The OD unit on either tranny is basically the same and will require the same amount of surgery to the floor of the car to fit in. I would go for the A518 (or as it was later called 46RH, you don't want the 46RE as it is computer controlled). All V8 pickups and vans had the A518 and very few RAM pickups had V6's so it might even be easier to find an A518.

I would think you could pick up an A518 for a $200 at the bone yard and basic rebuild kit is about $300. There are things you could beef up but you wouldn't need to do as much as you would with the A500.

"Not to mention give me more gears than the 500"
While technically you could create 6 different gear ratios with the GV unit for all pratical purposes it's just an OD unit the you engage and disengage on top of 3rd gear.