Paul,
You are correct, fill the all sonobuoy tubes.
Early P-3Bs also had a search light, but the Argie one does not use it now so you don't have to worry about it. You can also leave off the 2 sensor pods that hang between the inboard engines, they were a P-3C fit.
Externally that is it, though I do note on the Argie P-3Bs that there is some sort of dome on the aft section of the sonobuoy panel, I am still trying to figure the actual shape.
Finally if you want to be pedantic about the 1/144 kit, it comes with an Electra windscreen (4 panels) where a P-3 windscreen has 5. You need to file down the centre windscreen pillar to make a 5th flat panel in the center of the windscreen.
I did it with my RAAF ones
You might wonder why I have all this info? I have the 1/72 sheet and have started working on mine