67 GT500
Thinking about options to stop running 4K RPMs on the freeway on a regular basis......."
In your shoes, this would be my preferred order of options:
1) do nothing. your car is not being damaged, or subject to accelerated wear by running it @ 4,000 rpm. If the trip requires it, your BB can run all day long at 4,000 rpm and then do it again day after day. It's not abuse, it's exercise. I like highway cruising at 3,500 to 4,200 rpm and have spent many full days traveling running those numbers. No need to downshift when passing, and the engine is making beautiful music in the midrange.
2) change the rear end gearing to 3.50. just buy a complete 3.50 pumpkin and in 2 hours you will be done with the swap. That will drop the rpm down to 3,500 more or less, which may be less objectionable. Your low rpm torque will not dramatically be changed.
3) a 5 or 6 speed swap is giving up too much of the 1960's character of the car for my liking. You might as well go al the way and just put all Shelby Mustang sheet metal on top of modern underpinnings and be done with it. However, many people love it. IMO, driving modern cars has ruined their perspective.
In other words, they have gotten soft.
Z