Back in the early 80's I went from 3.91 to 4.56 gears in my 71 Cuda. It was a 440, 10.9 compression with mildly ported 915 heads a Crane hydraulic 528/552 and I think 246/252 @ .050. It had a Torker with a vac.780 and a 727 with a Select Performance 3500 stall 10" converter. If I remember correctly the car picked up about 2 tenths with the gear change. It was all steel with full interior(see pic.). I was shifting at it 6,400-6,500 and I think going through at about 6,800. Best ET/MPH at that time was 11.72/117. Tire was 11.5x28.5x15. It drove good around town but you couldn't really do much highway driving. If you don't do to much highway driving with it I think I would give them a shot. Shifting that thing higher and going through higher has to be worth a couple tenths with your combo.