well, according to MAME, pacman's native resolution says: rotate="90" width="288" height="224"
but with the rotate in there, it's actually 288 high by 224 wide, or at least that makes the most sense to me since it's a vertical game.
Could you 2x the resolution and set it to 576x448? Can you monitor do 576 in height? (since it's a horizontal monitor and that's the Y axis for the game)
[wow, this stuff is like thinking about time travel]
That'll still give you some vertical bars on the side, but should at least fill the screen out more and give you the same aspect ratio.
i admit i'm pretty much talking out ---my bottom--- here, since my system is already a vertical monitor and only does resolutions in the pac-man/frogger era anyway, so mine just worked.