Just speaking off the top of my head without any hard experience:

I would imagine that power line harmonics wouldn't cause too much of a problem...I doubt that they would be at high enough intensity at 72 MHz to be an issue. My guess is that the biggest issue would be that the power line could act as a giant antenna, and if there are any transmitters not too far down the line and close enough to it, the power lines could carry the signal some distance. This is where good grounding could make a big difference.

Of course, that's just my guess and my area isn't in Power Systems, so I have no idea if the lines have any sort of other issues. My area is in RF but I am only a grad student and not a practicing engineer so I could be wrong.

And if you actually fly into the lines and short two or three out, you will have a nice vaporized airplane pretty quickly and interference will be a moot point.