such a configuration will lead to all games more or less stretched to 240p and cause some horieontal stretching, resulting in some columns that can be 2 pixels wide, sometimes 1 pixel wide ... this should be quite visible on horizontal scrolling ...
Regarding Marvel vs Capcom, you'd need to understand how its MAME driver (or FBA driver) is made. I guess it's a CPS2 game ? so you'd need to write a hdmi_timing on your own, or see what's been done so far. This is what we're working on. I have an untested modeline for CPS3 that could suit your needs, or you can check http://sommerp.net/doku.php?id=raspberry_pi_arcade
Then you can just add for testing in your recalbox.conf fba_libretro.videomode=hdmi_timings 384 1 0 11 28 224 1 30 0 30 0 0 0 60 0 6400000 1
(fba_libretro being the system name) and then play around with porch to reach the result you'd want. So far recalbox has no mecanism yet to set a per system+game custom resolution, just a per system one