Glad to hear you sorted it out and saved the trip to the rebuilder. I didn't see your post about the assembly sequence question before you got it figured out. Sorry.
All that being said, I have a lot of Chevy and GMC original shop manuals and parts books. I found that the GMC books have a lot more detail when it comes to major component repair/rebuilding instructions with good diagrams to boot.
I've got a '47 GMC Shop Manual for the Model 300-450 trucks which use hydrovac units. The 300-350 models use the smaller 7" diameter unit while the 370-450 Models use the larger 9-1/2" units. Perhaps some of the GMC guys can comment on if the GMC smaller unit is the same design/compatible with the Chevy 7" hydrovacs??? If they are the same, I could try and share specific hyrdovac info to help out further.
