Many thanks to you all for your posts. I am a new member of this forum! I know this is a bit delayed but I am writing in the event that you may have found information since the original posts regarding the P-47 you seek.
My Uncle, 2nd Lt. William McGowan (spelled with an "a") 366 FG / 391 FS and the pilot of P-47D 42-76368 crashed in a field near Moon-Sur-Elle (I have been to the site) north of St. Lo. He is buried at the site of the crash. I have been unable to find specific information regarding the cause although I know that was a very "active" area on D-Day. Were you able to find any additional information? I am going back to the site with my mother, Lt. McGowan's younger sister.
I also found an article with additional information regarding pilots Martell and Shipe, also KIA on D-Day as you noted.
http://www.washingtontimes.com/news/...-path-for-gis/
Thank you!