Cutting axles

As ^^^ points out, you'd have to narrow the housing first!.. but I think you know that and are asking how much you can cut off the axle ends and still have enough spline.

As others have noted, a different backspacing on the wheels might be the better route, or a different model years rear axle assembly that may be a different overall, drum to drum, width. Doing that right now myself on my '64 Dodge 440, taking out the '64 rear end and putting in a '66 that is 1.75" narrower drum to drum!