46 "(your drum to drum measurement) minus 39" (your leaf spring outside to outside measurement) equals......7" total, or 3-1/2" per side.....subtract 3/4" per side for clearance. You total backspacing with tire on rim should not exceed 2-3/4".....unless you add spacers that effectively widen the total measurement to compansate. In your case for the 3-1/2 BS, you need 3/4". I do not like to use spacers, especially over 1/4", but your opinion may differ.
