Using 8 3/4 with /6

IMG_3362.JPG IMG_1527.JPG Hi,
I'm new to the forum, I have a 71 Plymouth Duster with a slant and I have a question on something I'm looking to doing to the car. Currently I have 7 1/4 rear end and I want to swap it with an 8 3/4 but I'm not sure what exactly I would need or have to replace when I go to do this. Also what would be the best gear ratio for daily driving that's not a slug off the line, I was thinking 3.55 but I'm not quite sure. Im still only 16 and have the car for just about 2 years now so any information would help.