How hard/expensive is it to switch out gearing on an A-body?

3.23 with an 833...
The OP lives in Oakland..right across the bay from a lot of hills.
My 340 Duster had a 3.23/833. Fine for up and down the freeway, but a cloud of smoke and slip driving in San Francisco. Keep your 3.91, IMO.

Sorry, but CT and NJ folks can't imagine SF hills. 'Bullitt', the movie: Yes, it IS like that!

DO take note of the thrust button comments. My 'green' bearing Mosers did NOT fit in a button-equipped diff.