While the Differential is out if the car, I thought I should replace the centre pinion bumper.
This is it:
https://secure.amesperf.com/qilan/Detail_Web?part_num=FS304P&order_number_e=NTA0ODExMA%3D%3D%0A&web_access=Y

It is a pull through type bumper and the slot housing is welded to the body so how do I pull it through the slot?

Also, does anyone have the dimensions on an original one? The one on my Firebird was missing when I bought the car so I have never actually seen one.

You should be able to push it into the slot.

Just use a little vasoline or bearing grease on it before you try.