Hello Greg- I'd seen replications done of the coastal artillery done for the National Park Service in 1968. The material used was cast iron, old engine blocks. These pieces are non-firing and are located at the fort at Ferandino Beach, Florida. I doubt you could get the funding for such a project these days, tommorrow being a major consideration, never mind yesterday.
Find a gun as you wish to replicate and measure it. Make the drawings and contact the Paulson Brothers. Those folks have been of help in my research of the 13 inch bore Seacoast Mortar, Model of 1861. Dean Thomas, author of several texts regarding ordnance of the period might be of help.
Brooke's journal might be worth a read before you do any work. Please keep posting regards progress of this project.

Ironclads and Big Guns for the Confederacy:The Journal and Letters of John M. Brooke Authored: George M. Brooke, Jr. ISBN:1-57003-418-4

One of the Brooke's, a 6.4 inch bore, I believe, is on the grounds of the Naval base in Washington, D.C. This one came from the C.S.S. Tennessee.