Frame specs are below for the SB. It doesn't have the measurement you're after but it does have two different ones you an use. The 43.03" and the 72.49" measurements from the reference hole just past the trans cross member location shown in the first photo. 2nd and 3rd photo go together and you could use the N or O measurement.
I just installed that chassis brace this weekend and had to open up the holes on one side of the chassis brace to allow the bolts to be 1/8" rearward due to the same issue. I shortened my frame and one side was just slightly 1/16 - 1/8" longer than the other when welded back together. I finally got it in there .
