wide wheels & tires& backspacing?

[7434gmc]

wanting to mount 33x12.50x16.5's on the old factory g.m. 16.5x 9.75 rims..and or 285/75/16's on brand new "wheel vintiques" 16x10 factory style steel rims on the back of my 74 gmc 3/4 ton 2wd 8ft stepside( am converting to stake body)....the old factory16.5x 9.75 steel rims supposedly have 4.5 back spacing,...not sure on how to measure backspacing on vech ..if i can't find (3) stock 16.5x 9.75 factory rims that "should" go on without any issue's..& go with the new 16x 10's ..does anyone know what back spacing to order them with?...i have newer 90's 16x6.5's on the front with 245/75/16's for the front now,..if it makes any difference?