I know that the big bolt axles add width, so you are wider than a stock A-Body axle. My buddy's 8.75" A-Body axle with green bearings measured 58" drum to drum. Not sure how much of that additional width is due to the green bearings, if any.
With a 10mm spacer, I would calculate a little over 1/4" clear (58" plus 2x .39" for the spacers minus 2x 6.36" backspacing minus 45.5" for the spring out to out distance leaves .57" clear total). Calculated the backspacing by taking half the section width plus the offset (275/2 + 24) and converting to imperial units.