I could see where 3.23's and 15" tires ain't giving you the low end you'd like. the 904 low gear would be going from a 2.45 to a 2.74 so your first gear overall ratio would be 8.85 (2.74X3.23) as opposed to 7.91 (2.45x3.23) plus I think 2nd gear might be a bit lower with the low gear set also but I ain't sure on that. I would make sure the eng is tuned right including the adv curve with a dual plane intake and the right carb then consider what guys have posted on more in depth changes. If you ain't out on the interstate alot I dont see a problem with 3.91's even. going from a 3.23 to a 3.55 is going to up the RPM's ~10% in all gears and the 904 low gearset is gonna up you ~12% in low gear. Just me I'd go with the low gear set plus a top notch converter if I took the trans out plus 3:55's plus super tune the eng & the components on it EDIT Along with the low gear/better tc, a 3.73 742 clutch sure grip would be my hands down wish list. I had 4:10's with 14" tires in my first Mopar in 1975, a 440 GTX 4 sp convertible & that was toooo low. The next year I drove it to California & that was back when the speed limit was still 70