According to mame .103, the correct resolutions for Ms. Pac are:
224 x 288(V) @ 60.60Hz
On my AVGA2, Ms Pac is overstretched a bit, cutting of a section of the bottom. I have not yet figured out how to fix that.
You have to adjust your monitor.
When I set up my cabinet, I used Gavin Benson's AVGRes utility to set up all the resolutions without hardware stretch (I manually add it to games that need it on a case by case basis).
Then I adjusted for "vertical" games... I ran pac-man and adjusted my arcade monitor so that the image just fits on my screen without cutting any off the top or bottom.
Adjusting the monitor to fit vertical games has the side effect of very slightly squashing horizontal games. There's a black area at the top and bottom, kinda like a letterboxed movie with much smaller black areas.
It's very hard to notice unless someone points it out to you and I think that it's totally worth it to be able to run vertical games without resorting to hardware stretch which tends to make them look crappy.