Cliff:

If you're looking to do the monoposto to biposto mod to the FE (as in your other post), the header itself is the same and doesn't need to be replaced. All you need to do is replace the OEM high mount cans with low-mount slip on cans and passenger pegs.

The reason why slipons for the 900ssie bike will not fit the carbed 900ss bike is that the mounting locations (from the pipe to the passenger peg) is different. The 900ssie pipes and cans will technically slide in to the 900ss headers but you'll have a hard time mounting them on the bike - unless you create your own mounting brackets.

This is what I did:
http://www.geocities.com/edwyun/monotobi.html