Hi Lionel
The engine nr. is on a boss just near the distributor on the drivers side. You may get lucky and find the engine nr. in the AWM 126 ledgers.
https://www.awm.gov.au/collection/C1424610
I think you need the NASCO Australian Army 1940 parts book.
The Holdens tag on the firewall will have a date code i.e. A0 = Jan 1940 and so on . Do you know if the truck ever had the 'overseas wheels' fitted , the 9.00-18 rims ?