dancing speedometer

["Bruce.Milne" <bruce.milne@fulcrum.com>]

My '71 B (No OD) speedometer bounces wildly at any speed, making it hard
(read impossible) to get an accurate speed reading.  I've replaced the
cable to no avail.  I've determined, then, that I need to change the
pinion at the tranny end.  Is this a tough job?  I have a donor car from
which I can pull the part, but I haven't been underneath to see where it
is.  Is there a seal that will need to be replaced?  

Thanks in advance for any opinions.

Bruce Milne
2 1971 B's
Kanata, Canada