Funny, but all the games you mentioned use analog stick, which isn't going to work on your cab with the d-pad...or at least won't be much fun with a happs 8 way.
On the control panel design, I went with 2 rows of 3 buttons (ala street fighter), and then attached 2 more buttons arcing down off the lower first button (for Soul Calibur and other 4 button neo-geo type config).
I believe the layout for Dreamcast is, LRA on the top row, XYB on the bottom row, and AB again for the two lower ones. This way ABXY is in a 4 button neo-geo style layout, and the rest are in classic Street Fighter layout.
Using the same control panel with my Playstation2 setup, the row of 6 buttons becomes L1 R1 L2 on the top row and X Y R2 on the bottom row with AB again with the arcing buttons on the lower left.
Here is a pic of one player's layout: