I'm sorry, Mike, but I do not having any information on that aircraft or any ideas about where to search.
I got a hit on N9599C at this site, which has a lot of information about Avenger tankers, but it does not add anything to the info you have:
https://nbavengers.com/489-2/u-s-mou...d-wyoming-new/
These two sites have images and info on Avenger Air Tankers that flew for Air Tankers, Inc., but nothing on your Dad's airplane:
https://nbavengers.com/gldx-glex-ser...s-norfolk-b19/
https://nbavengers.com/gldx-glex-ser...s-norfolk-b16/
This site covers fire bombers and lists BuNo 85594 (N1366N), but does not list spray tankers like N9599C.
GEOFF GOODALL'S AVIATION HISTORY SITE
I wish I could be more help. I will keep an eye out for any info on N1366N.
Don