How far along are you in terms of cab building? Do you have your cp wired?
You need a KVM with USB and sound. 2 mad catz sf 4 pads, an xbox 360 VGA cable, a female to female VGA adapter, a female to female 2.5mm I believe, sound adapter and a RCA to 2.5mm adapter.
Buy 2 mad catz sf4 pads and solder them. Ditch your iPac or key wiz or whatever, the 360 pads are recognized as joys in windows with drivers you can get from MS. You CAN wire up an iPac and the joysticks together, that is what I am doing, but this just adds complexity.
Connect pads to controls, plug controls into kvm. Plug KVM into computer and connect the xb 360 to the KVM with above mentioned adapters. You are good to go