Originally Posted by EragonSuperH
I recently purchased a 1979 GMC K25 and I'm trying to figure out the GVWR. There were 3 options for that model and year, which were 6800, 7200, and 8400. IIRC the 6800 was only available with the standard rear brakes and mine has the HD 13 inch rear drums. It also has 7 rear leaves which leads me to believe it might be 8400 but I want to make sure. It also has 3.73 gearing and a 14 bolt full floating rear with a Dana 44 front. Any help is appreciated.
If you have vacuum breaks then you have the 6800 rating.
If you have the "HD power breaks"= Hydroboost. Then you have the 7500 or 8400 rating.
Due to your rear axle ratio I would go with the 7500 rating.
I believe the 8400 rating was only offered with 4.10 or 4.56 gearing and a few extra codes.
This is for 78 but is a good reference for you.
Page 13 and 14 for GVWR.