Question on backspacing on rear wheels

Hello guys
I have a 73 Duster and am looking to use 15x8 rallies on the rear with a 235 or 245 60 rear tire. Im having a hard time determining if I should use a wheel with 4 inch back spacing or one with 4 1/2 inch backspacing. I dont want to run any type of air shock or air suspension just simple rear set up with no tire scrub.
Also I have found one company that sells the rally wheels new but they only offer a 15x8 with a 51/2 back spacing. Any idea on who sells these wheels?
Thanks in advance
Chris