I will look for some measurements for you tonight. Do you have the truck frame accessible? There should be holes in the frame for the cross sills.
A '55-'59 3100 is not what you are asking about... However, I have a 1955.2 and here's the factory assembly manual (FAM) page of a
'55-'59 showing the cross sills.
Again, I understand you are asking about a 1953, but maybe this will help (?).
Brad