71 Dart Swinger rear tire fitment

i just purchased a 71 Dart Swinger. it has 15x7 rallys on the rear, not sure of the tire size yet cuz i havent had the car shipped to me yet.. my question is... can i put a 15x8 rim on the rear?? if so, what size tire will fit under it? it has stock suspension and im not sure if its 4.0 or 4.5 bolt pattern... any input would be greatly appreciated!! :burnout: