As far as an18x8 matrix does it really matter?
IMHO The best thing to do is download either ghostkey (dos) or virtua keyboard (windoze) & map the inputs.
http://www.arcadecontrols.com/arcade_downloads.shtml#UtilitiesYou can then solder leads to your pcb & check what inputs cause key blocking & ghosting & what do not.
Doing this, I was able to get one of my projects, using an old gateway keyboard pcb, to be ghost/block free except for fire button #6 on the player 2 side.
I can hit any diagonal with both sticks & press all six buttons on player one side as well as the first five on player 2 without any conflicts whatsoever.
For the price of the keyboard ($0) I can live with this.
It will take quite a while to test this, but if a keyboard hack is your choice, then this is what you must do or else you may have a situation where you can not move up when player 2 presses button one etc...
Of course you will have to remap the default inputs in mame to match your control panel inputs.
I do have to agree with tom61 however, a gamepad hack or an ipac or keywiz is the way to go.