Well here it comes do a search - then I suppose you can ask, WHY "I wondered why the baseball was getting bigger then it hit me." Remember, I'm just a fng-lb+1
Ill repeat the scuttlebutt that I was told years ago but never verified. The 56 box was preferred because the 56 was the first year for the dished steering wheel giving it a shorter shaft and tube for a better fit than the 53-55.
Tommy's right, the 56 Effie box & shaft is shorter (35 3/8") than 53-55 (36 7/8") because of the all new "driverized" cab for 1956, which featured a safety-designed, dished, steering wheel. (I'm an automotive marketing guy & even I don't buy that line!) I can't speak to specifics on pre-53 or post 56 boxes, but know that they are not interchangeable w/ the 53-56's. Check out Mid-Fifty F100s in Arizona for parts/pieces on these (www.midfifty.com). Hope this helps.