I don't know about the cen diffs but they will take BP diffs.. They just require flm cups if your using a flux. Other wise i do believe they are easy to drop in. The shimming i believe is just to stop the slop in the bulkhead but if you want that slop in there just don't shim them.
At the moment i am waiting on FLM outdrive cups to finish my flm hybrid bulkhead conversion. I'm sure when I'm done ill be posting in my flux thread if you'd like to follow.
although the diffs for Savage X, XL, Cen GST & HPI Bulletproof have different tooth count, they all have very similar diameter and will fit your bulkhead.
thanks for the reply's looks good i just wasnt sure because there new bulks are set up for cen or buggy type diffs you guys have the new ones there the super hybrid buks Part#: FLM40020 and they say no shimming needed!