After a little further research I have found the following:
There is no recorded birth certificate or death record in the online Orleans parish database for a William David Cassidy. There is a David Cassidy who died at 38 yrs on December 31, 1870 - located in Volume 51, Page 94. ??
In the 1851 New Orleans directory there are the following Cassidys: Henry, James, J.P., John, John, Patrick, Thomas, Mrs. M, and there is a William "Cassiday" listed as a stabling and living at 53 Gajennie.
In the 1859 New Orleans directory, there are three John's, James, H., and Hugh.
These are the only directories I have presently available. I don't know if any of this helps but it's all I could find.
Good luck. I will keep you posted if I come across anything else.