Does anyone know what thread size is used for the carb that holds snorkel/air cleaner down with the wing nut? Also is there a propper name for this?

 

It's called a carburetor air cleaner stud or air cleaner stud, in the parts book it's called "Stud ..cleaner to carb".
The 2 barrel is 1/4"-20 x 2-1/2", 4 barrel is 1/4"-20 x 4-1/8".
They're both the same part number, the service part is a 9" threaded rod you cut to length.
The part number is 3785256 if you want the GM one, they're all over the place if you do a search, GM dealers probably have them.
At GM direct they're 5 for $3.57.


http://www.gmpartsdirect.com/results.cfm