Opps! Your is an Advanced Design, my bad. This guy sells a complete body mount kit... and should be able to answer a lot of your questions.

http://www.ad-engineering.com/

http://www.jagsthatrun.com/ has parts and info that can help too.

The GMC board and site has info too:

http://www.oldgmctrucks.com/

Jim

Good luck with your project!