Seems like you are dealing with a 66-67 B-body? Not sure but if that is true, your numbers are right by plan. I've got 7in wheels witn 4.5 backspacing on 275/60/15 tires with 5/8 in on both sides. I think that, by your numbers, you will need 8 in with 5 in back spacing to fit evenly. Problem is little or no production wheels are that dimension. That is last time I looked. Also going from 7 to 8 inch rim will net a negligable change in section width. Just my 2 cents here. I believe that you are looking at this correctly. Good luck