ooh that's bad. if its slipping it could be toast. if you look up the filters the 400 is a odd shape like the state of Illinois. the 350 pan is square with a corner cut off.
if you pull the pan be careful not to disturb the crud pattern in the bottom. if you see large chunks of metal look too see if they have piles on the back of the filter also. if you refill it and it doesn't move its most likely toast.
our resident tranny expert should be along to give you some pointers when he sees this thread