Opinions on 4 speed with OD

I've got 3.23s in my 67 and it runs 2500rpm and 60mph with a 340. My 71 duster im building has the cast iron OD box going in with 3.91 sure gripand I also have a 3.55 full spool center I can swap back and forth and also another 340 car Im mixed now on weather to run the od or standard non od....