@cyclair When I looked at your linked .rc file, I see:
These should be changed. Now, if you are saying you already tried that, I would suggest changing from AdvanceMAME 1.4 to the newer AdvanceMAME, which is 3.7 at this point I believe. It uses the same romset, so no extra work there. But, the config for aspect ratio changed. The current way to capture this would be:
display_aspect 16/9 according to the current documentation.
If you also set
display_expand 1.78 this will stretch the image to fill a 16:9 display. I just tried it with Gauntlet II and it was fullscreen.
OPINION FOLLOWS: It looked awful. The game was filling my big TV, but it was squashed. Round things look like ovals and everything is just distractingly stretched.