If you could post a dia of the Francis switch it would be helpful in drawing you the modifications. But you are on the right track.... Vic is correct, the key switch controls power to the fusebox ....... switched 12v side.
Depending on how the switch is terminated you will need a couple of relays to control certain items....like the switched 12v side of the fusebox, and pwr to the coil, your starter button should get the purple wire (if Francis's kit followed GM color codes) that goes to the "S" terminal on the starter etc....
I assume your existing key switch is on/off. Would you consider puting the new key switch in and use that to control the on/off functions and still use the starter button to engage the starter, that way you wouldn't have to do any modifications.