Hi.
You can find some information about Williams on the site New Zealand Fighter Pilots Museum
http://www.nzfpm.co.nz/article.asp?id=williams_ga
He is also included in the books
Aces High by Christopher Shores and
By Such Deeds by C.M. Hanson.
67 Squadron is mentioned in the official history: New Zealanders in the RAF Vol 3:
http://www.nzetc.org/tm/scholarly/tei-WH2-3RAF.html
and the book
Air War for Burma by Christopher Shores (this book covers the period from June 1942 to the end of the war). It´s very difficult to find any information about this unit and you already have the best books I have found sofar (Bargh´s book and Bloody Shambles Vol 1 & 2). If you wants to have more information about the daily operations I think you need to get a copy of the Operational Record Book even though it will probably not be complete. If I rembember it correct the original was lost during the retreat in Burma.
Cheers
Peter