Rear drum brake sizes????????

I am currently trying to fit wheels for my 72 dart Swinger and I dont seem to be able to fit the same size rims and BS as other folks. i am wondering if this can be due to the type of drums I have. can someone educate me on the different sizes they made? I have a 8 inch rim with 4.5 back spacing with a meager 225/70 x 15 and I am slammed to the wheel lip. I am thinking that maybe I have lost some space due to the drum size. thanks