I think I understand what you want to do : Install the Multi-Williams in your Williams cabinet without replacing any of the wiring or removing the connectors on the facory wiring. Correct?
It seems your thought is to mount a female 56 pin edge connector on a circuit board and then add all the diffrent connectors for the Willams wiring harness and wire the two together on the circuit board. Does that also sound right?
I have another idea that might be a little easier to impliment. Buy a standard JAMMA wiring harness and cut it to having only about a foot of wiring. Then solder the wires to the type of connectors that Oryk Zinyo is talking about. You will save yourself a lot if time and have fewer possible failure points that way.
Brian.