Using 8 3/4 with /6

Welcome to the site and that is a very nice Duster. I have a 7 1/4 Sure-Grip in my 66 Valiant and have no issues with it. Yes the car is slow, but I don't care about how fast it is. I just have fun driving it. I think that you should keep the 7 1/4 and just gear it up to 3.23's or 3.55's. It would wake it up a bit.