Sandy Parts,
The total USMC fleet of KC-130J is planned to be 47 aircraft out of 104 aircraft for the USN + USMC - deliveries are ongoing.
The latest plan is for 10 aircraft (5 per MAW) to be modified to accept 6 kits (3 per MAW) so I assume the kits will usually only be changed over when required by the aircraft maintenance plot.
The Right to Bear Arms: Gunship Kits for America?s C-130s
I don't imagine there is too much to connect up between the removable kit and the aircraft once it has been modified for the HARVEST HAWK role with pre-installed wiring and junction boxes / connectors - presumably connect the WSO pallet wiring to a point, the Gunslinger launcher on the ramp to a point, and the sensor ball and the Hellfire pylons to the aircraft (but I'm just guessing).