RAN FAA COIN conversion over Vietnam?
That would depend on where it would be used. If operating as part of the "Market Time" coastal interdiction mission it would most likely perform similar tasks to the Martin SP-5 Marlin. This consisted mostly of long flights along the coast line looking for NVN/VIet Cong water craft trying to infiltrate SVN territory.
Another possible mission for the Gannet might be that of surveillance along the HCMT* in Laos and Cambodia carrying the AN/APS-94 SLAR** attached to the fuselage [OV-1 Mohawk SLAR system]. Other surveillance equipment might be an downward looking/viewing IR*** system [AN/UAS-4 "Red Haze" infrared (IR) sensor system], LLLTV****, or NVD*****.
I would imagine that any of the above missions would require additional fuel tanks to be carried under the wings to provide the desired range and endurance. The surveillance equipment would be carried within the confines of the bomb bay (with room to spare for other stores).
*HCMT = Ho Ch Minh Trail
**SLAR = Side-Looking Airborne RADAR
***IR = Infra-Red
****LLLTV * Low-Light-Level Television
*****NVD = Night Vision Device (aka "Starlight Scope)
Links:
Wikipedia > Grumman OV-1 MohawkGreg Goebel's Air Vectors > Grumman OV-1 Mohawk