I assume the glass is for a '53 as your name suggests?
By how much do you think the glass is too small,the rubber can compensate for small discrepancies.
The rubber for the rear of the cab is an 'H' section,you would need make a template from the aperature and subtract the thickness of the middle part of the rubber between the glass and body grooves. Allow slightly more than the rubber thickness to give yourself some leeway,you do not want it too tight as laminated glass isn't as forgiving as tempered.
Have you contacted the supplier to see what they suggest?
