Axles and wheels

NEED HELP..Son wants to put on wheels[73 duster] and i want to go from a 8 1/4 to a 8 3/4 first..its a body 8 3/4 ,so it has sbp axles..i have a set of 10 x 2.5 backing plates from a truck..if i buy axles made for these brakes dimensions.do i have to reconsider the backspace on the new wheels or will the back space i want to buy now work..will new axles push the new rim in or out and do i now have to consider hitting at the top of the wheelwell,all help appreciated..j