Comeaud,

Welcome to the world of older GMC large trucks.

I have a 1966 GMC HM5500 and found the following site helpful in decoding the VIN. It is not easy to follow, and didn't work for my truck's wheelbase, but was otherwise helpful.

http://6066gmcguy.com/TwoTon.html

Finding parts has been a challenge for me. In some cases, I have needed to figure out what fits from other models. It helps to get a GM part number that can be cross-referenced to parts store numbers, such as NAPA. And there are dealers who specialize in some part types, such as master cylinders.

Good luck!

JRMunn