The twin and triple bulkheads are very different. Not all triples are the same either, and neither are all twins. The seem to be dependent on what suspension is under the sled, Xc 100, XC 101, Xtra 10, X 12 ect. If you get a bulkhead off a different chassis your almost certainly going to have to get the engine straps or plate with it, because more than likely yours will not work.
AS CK said as well CRC and Non CRC and inboard and outboard trailing arms are other differences.
The steering post changed over the years too.
Another thing to keep in mind is the chain case changes over the years.
In my experience they are different enough that your best off to get one as close to what you have as you can get.
