8 3/4 Sure Grip ratio change

I can tell you from experience.
I had a Challenger in my late teens, I would change the 3.23 out on friday after work, stick the 3.91 in for the weekend and sunday evening stick the 3.23 back in.
3.91 suck on the highway engine is just screaming, but are way fun around town.
If you make the metal brake lines on the rear end long enough, you can slide the axles out enough to change the center sections without bleeding the brakes.