Ditto, 350 hp an automatic ? Stick with the 8" rear. Just avoid drag strip launches with sticky tires. There were only two choices for donors for 9" rears, one is the 57=59 Ford 9" and the late 70's to 1980 Granada/Monarch/Versailles 9" rear, this one came in both disc and drum brakes, but the disc rear is 1" overall in width more than your 8" rear. Don't know about the drum unit, never had the opportunity to measure one.