Is there a site where there are diagrams or lists of what size bolts/nuts that are used in certain areas? I can't figure out the size of nut I need for the seatbelt housing, and some other things, and need to find out. Any help would be appreciated!

 

If you download the parts book pdf, it lists lots of stock sizes of bolts and nuts.

Camaro parts book

I am not sure where you can get the Firebird book any more, but most of the fittings are the same.