I ran a 3.91 SG 7 1/4 rear for years in a low 13 second drag car on slicks. The 7 1/4 SG rears are actually pretty strong. That said, getting parts for the SG units (if you can find a sure grip) is almost impossible. I have not seen any clutch kits for several years, and most any 7 1/4 SG will need clutches. Also new spider gears and axle end gears are hard to find, cross shafts are impossible. So just be aware of the parts availability situation.