Hi Mark,
I have an automatic trans which is mid stream in being replaced. I looked at my tach, to remember what RPM 65mph is. I would say 3800 RPM is pretty close. When I get up an running I will confirm it for you. I have never heard about custom gearing used with automatics. I do know there are custom geared ,manual transmissions available for a price. I recently saw an ad for a 5 speed manual trans I think was made for a diesel.
Guy