1974 Duster wheels & tire size

I have a 1974 Duster, 6 cyl, currently small bolt pattern wheels, dum brakes all around. I am installing disc brakes on front, they have the bigger bolt pattern, need new rims. Also, have new rear with 4.5 x 5 bolt pattern. (8 1/4). So, I need to get all new wheels. I want to go 15s, what back spacing wil fit best? And, what tire size will be best? I'm not going to be a speed demon, but, would like it to look and ride nice. It's my first car, I'm 16. It's a sort of project car, and eventually hope to be my daily driver. Thanks for any advice.