I am surprised you didn't run into problems hitting the leaf springs with a 4 1/2 backspace. I sure did with a 275 and a 255 , and this was on both sides , not just one side. Is your car small bolt or big bolt ?? Mine are 15 x 8 steel wheels ( small bolt ) with a 4 1/2 backspace on a 70 Duster ( with a small bolt 8 3/4)
By the way .......Very nice Duster , I like the not trim ring look...
Frank