Hi...
1. did you try NARA's contact in College Park, MD? The fire burned individuals' files and some morning reports, but NOT unit records. They are at College Park, MD.
Contact them via their web site.
2. I found the following:
August 2009 The Coast Defense Journal Page 73
505th Coast Artillery (AA) Regiment (Mobile) Activated in AUS 6-1-42 at Cp. Edwards, MA, and moved to Indiantown Gap Military Reservation, PA, 11-29-42 and staged for overseas deployment. Departed NYC POE 12-8-42, arrived England 12-13-42. Moved to North Africa 3-4-43 and landed in Salerno, Italy, 9-9-43. Inactivated Santa Maria, Italy, 3-14-44. HHB redesignated HHB 505th AAA Gp; 1st Bn redesignated 87th AAA (Gun) Bn; 2nd Bn redesignated 900th AAA (AW) Bn; 3rd Bn disbanded.
3. Given that the unit was reorganized, it is likely that many individuals were reassigned to other branches. That's how my father went from AAA to Combat Engineers.
4. (Maybe your best bet) You should also contact the VA and request your Dad's VA file. I think they are separate from the files in St. Louis. You are entitled to it as next of kin. That file has all sorts of stuff in it (My father's had an evacuation tag from an incident when he was on active duty and was hurt). IT ALSO should contain enough paperwork that you can trace your father from unit to unit. I had access to several files during my Ph.D research...and was able to do so for several men...and these files came from various locations across the country. They were sent to my local VA clinic to allow me access.