What resolution are you trying to fill?
I run it at 1920x1200 on my development PC, but admittedly the bars on either side do not bother me much
On my cab it runs at standard 15Khz 4:3 aspect ratio of course.
One of the easiest ways I could make it fill 1920 x 1200 (16:10) is simply to extend my background image of the circuitboard to fill the black space. I don't think many mods would be needed to the layout .nut file in that case.
Of course, when your setup goes to the screensaver videos, it'll still only be 4:3 with bars on either side because that's how the original games were?