I think what I'm going to do is go back to the junkyard and return the booster I bought and pull one from a 2nd gen Astro, complete with the master cylinder and prop valve. I have determined that my stock cylinder needs the long pushrod which is not what the 2008 booster is. Here is a link to Hydratech's website - at the bottom they have pictures showing the differences in the pushrods. I wasn't aware of these differences and I agree that it makes sense to get a complete unit. I had read in another thread that maybe the 2nd gen Astro units wouldn't work because of the downward angle, but I've since found a thread by Captainfab for his hydroboost brackets and plates and it looks like several people have used the 2nd gen setup without issue.
http://www.hydratechbraking.com/GM_67-72_CK.html