Stock 318 w/ 3:91. Opinions?

Hey all.

I've got a '73 swinger 318 with a 904 trans & 8.25 rear. I've been keeping an eye out for a sure grip unit with hopes of swapping the gears out for 3:21 (2:96 now). I figured with a basically stock 318 (4 bbl & exhaust) that the 3:21 will be a nice balance of throttle response & freeway speeds. A fellow member has a sure grip avail w/ 3:91. I was hoping to get a little feedback from anyone who has run either the 3:21's or 3:91's in a similar setup & can give some real world advice.

The Dart is not my daily driver, I don't race, I would say that "spirited" driving & cruising would probably fit for how I drive the car. I seem to be mostly light to light but I do take it on the freeway & would still like to be able to hit 70-75 MPH at a reasonable RPM. Right now I have a stock 318 w/ an upgrade to a holley 600 & dual exhaust with an x-pipe. Future upgrades include a new cam & 340 manifolds or headers and some mild head porting.

Thx in advance.