Should I swap the 3.91's for 3.55 or 3.23

The 3.55s are a great choice. That is what I have in my 69 Cuda with a warmed over 340 4 speed. It does work well. However, I had a 340 4 Speed Cuda when I was in college (bought it new in 1969) with 3.23 gears, and it was AWESOME!!It pulled like a freight train, and I rarely got beaten in a drag race. When I power shifted into second gear, I would sometimes gain a whole car length and the in dash ash tray would pop open. I have considered changing my gears to 3.23s many times. You have to ask yourself what your primary goal is? There is no question that the 3.23s will be better for cruising and give you better fuel mileage, and smokey burnouts will still happen at will. But if trips to the track are in your future, then the 3.55s are the way to go.