I have been restoring a 68 coupe for about five years now to be a daily driver. I just recently got the car where its able to drive. I'm not by any means an expert on any mechanical device so I've done the majority of the restoration and had my uncle put a 302 hi output out of a 67 mustang and a 4 speed transmission that I bought from a guy that is a retired amateur racer. The problem I'm having is that I drive on an interstate at 65 to 70 mph. at these speeds the cars is running at a fairly high rpm and I would like to know if I need to put an overdrive on the 4 speed or do I need to upgrade to a 5 speed tranny in order to get the rpm down at those higher speeds.