Ryan, It's no problem at all. I am by no means an expert, all I can really comment on is what I have done. The best way to measure for the axle is to place the wheels and tires up under the car where they are going to be then measure. My rear measured 48" flange to flange, overall 53.5" with a pinion offset of 0.75". The spring perches are 36" however I would only get them tacked in position because you have to set the pinion angle (typically 3 degrees).
Are you minitubbing or full tubbing? if mini I would recommend your axle be at least 1.6" wider than mine, ( my backspace is 6") my tires are slightly closer to the frame than to the outer wheelhouse. (I'm considering a 1/4" spacer just to center the tire in the wheelwell a little better). I think it is better to error on the short side rather than long. It is easy to add a little, but a ***** to take away. If I were you I would go with a 55.5" overall dimension, but please double check all your measurements.
Hope this helps.