Hi caskad,
You may want to use the "CUSTOM" type in VMMaker.ini and then edit the monitor_specs line.
However, for your case better leave the VMMaker.ini as it is and directly edit groovymame's mame.ini with a line like this:
monitor_specs0 15625.00-16200.00,50.00-62.00,2.000,4.700,8.000,0.064,0.160,1.056,0,0,288.0,448
(modify the default "auto" value for monitor_specs0)
The values in black represent the front and back porches (right and left margins).
If you're getting big side borders, decrease this values a little bit and see what happens. I'd start with 1.5 and 7.5, for instance.