There is a great photo of the underside of the C-130J at post 19 on this thread: http://www.freerepublic.com/focus/f-news/1331873/posts
There is plenty of room to put in a center line bomb bay between the pods holding the landing gear. The cargo bay is also tall enough to accomodate 8 ft vertical tubes for the buoys.
You are probably correct that an updated P-3 would be a better bet, but a 130 conversion would probably be more suitable than a 737.
I've actually been on the MMA 737 prototype. Spoke at length with the reps (take that for what it's worth) and they made a strong case for its performance at low level. Plus, with newer technology, you'd probably not have to fly at 300 feet to perform a mission. You could hold at 1000 and stay a bit further out of MANPADS range. Of possibly greater value was the increased reliability and lowered maintinence requirements.
Oh, yeah, one other thing about the 130...assuming you COULD put in a bomb bay and 150+ PSLT's, you'd still have to find somewhere to put the antennas. P-3's have their sono antennas (with which we "talk" to the buoys) on the bottom, as well, as that's where the signals come from.
Point is, MMA would have changed ALL our tactics, due to its increased flexibility and adaptibility. A "new" P-3 or modded C-130 would have been more of the same we've got now. A single MMA replaced three P-3's.