I think the one behind the bell housing is the convex type that sits on a lip. If so, put some sealant on it and hit the centre to expand it. A rebuilder told me to place a hammer on the centre and then strike the hammer with another hammer. I have some photos in my webshots of mine before and after.
http://rides.webshots.com/photo/1316417356041873203xTDshC

I replaced mine when I had the toeboard cut out.


1951 GMC 1 Ton Flatbed -- It is finally on the road and what a great time I have driving it!
1951 1 Ton Completed


My Chevy Master 4 Door is on the Road!