That 53" you're seeing is for the body mount holes (26 1/2" x 2) I think. The crossmember bolt holes are a bit outboard from that, at least on my car. My crossmember holes are approx. 53 1/2" apart, definitely not in line with the upper body mount hole that is towards the center of the side frame rail.
I'm going to ask something stupid here, and please don't be offended (because I've done this very thing before). Sometimes when I use a metal tape measure, I'll start at the 1" mark, so I don't get involved with the metal hook on the end of the tape. I've occasionally forgot to subtract the 1" from the total measurement, scratch my head a few and then laugh at what I did. Any possibility that may have happened, because it would then put you right in the ballpark.