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.
I think that that would be the greatest engineering challenge. How to maintain the structural integrity while cutting a long hole in the bottom of the plane. The 130 does have the advantage of having been over-engineered in the slide rule days, so there is a far greater margin to work with than there would be in an aircraft designed on a computer.
I recall that a big question for the 737 modification was the loiter time.