Anyone shorten a rear to optimize a wheel/tire combo in an uncut A body?

I have a '73 Dart that came with a 7 1/4 rear and want to go with an 8 3/4. Considering a Moser fabricated unit but not sure of the length? I have Centerline Warrior wheels - 8 x 15 w/ 4.5 backspace and Mickey ET Street Radials. With the wheel/tire centered in the wheel well and as square ad I can get it, I measure 55 5/8" mounting surface to ms. I've measured several times and come within 1/16" each time. I've read where others have used a similar size wheel/ tire combo on a stock A body 8 3/4 (57 1/8") with no interference issues - maybe an offset hanger kit. Don't see how that can be since my measurement is 1 1/2" shorter? Wondering if anyone has done something similar and if my measurement is accurate? It's rather pricey and I can't afford to mess up. Any help is greatly appreciated.