The easiest and most secure way to MIDIfy a vintage poly synth is the use of reed relays.
This has been proven today by Francois and me :)
View with removed frontpanel - each key has a seperate pin, which is connected to -15V when the key is pressed.
In parallel to these key->gate board connections, Francois has led through the inputs via ribbon cables:
The backside of the synth with a nice view to the voice boards (sorry for the bad picture quality!)
Core Module + 6 DOUTs stu