Did you get the point about the serial number? Do you have a 1951 or 1952?

A GMC shop manual for your year truck would be most helpful to you. I only have Chevrolet.
I could only find a GMC manual for years 1955-59.
Frame section 11
http://chevy.oldcarmanualproject.com/gmc/5559mm/5559mm444.html

Search Stovebolt for more information. I found this:
https://www.stovebolt.com/ubbthreads/ubbthreads.php/topics/1164305/1.html

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."