There is a thread on this subject.I think 400 ex will interchange,but you have to use everything,spindles also.Maybe Boomer had some sort of do-dad that worked instead of swaping spindles,cant recall,hope this helps.
One thing to think of is if you get the ASR arms on ebay to know that they are made for 450r shocks. I have the exact models needed list and what 450r front shocks work incase you go that route.