Is this a modern LSD? One that supports LCACs?
http://www.globalsecurity.org/military/systems/ship/lsd-49-schem.htm
Hmmm, nope
http://www.ussthomaston.com/photos.html
You might see if the blue prints are archived some where.
Starting at Ingalls ship yard and tracking where the data went.
I know that when they close the Philidelphia ship yards, a lot of the files went to the National Archives in Philadelphia.
Also contacting the webmasters of these LSD sites might land you a plan soruce also.
http://www.ussthomaston.com/links.html
I did not see any plans for LSDs at
http://www.floatingdrydock.com/
Good luck on your research.