I just finished a 51 restoration last summer. The pump is not mounted under the back seat. It is actually mounted vertically behind the left side rear quarter back seat side panel. If you have the old pump you can see how it mounts with a bracket to the inner sheet metal behind that side panel. I believe the rebuilt pump I bought had the bracket on the pump. Let me know if you need the supplier info. It was an ouitfit out of Florida that advertises in Hemmings. They supplied the cyliners, pump, and lines. Very nice parts and the top works great.
Here's a pic of my 51.
Steve