Here is some reference info:

This link is from an IHC website. Zenith Bulletin Z-190-B for 28 and 228 models. Print date June 1950.
http://www.restoringcornelius.com/files/zenith_28_128_rebuild_manual.pdf

Zenith ID numbers for GMC with rebuilt kit numbers (sold at this site).
http://www.thecarburetorshop.com/TruckkitsG.htm#TkitGMC

also troubleshooting

http://www.thecarburetorshop.com/Troubleshooting.htm

"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."