Details on the 7660 kit are here:
but there's no full schematic.
You could certainly route the audio to the correct pins for the 700, but would it not be better, rather than modifying the leslie, to make a small relay board that goes in the organ, taking the switching voltage for the 215 and using that to power a relay that could give the 700 the appropriate grounds it needs to switch speeds.
Remember that this method won't give you main/echo switching, but the 7660 wouldn't do that either. If you want that, you can buy or make up another kit that would incorporate this switching. Can't remember the model number of the kit right now, but we did discuss this and find the solution on the Hammondzone Yahoo group a few months back.