I have a GM HEI distributer, the basic problem is that you need to make it missfire in order to drop out the overdrive. The part of the stock kickdown switch that does this when you have points works backward to what you need to happen with the HEI.

This is the diagram of the stock OD in my '62 below.

This is what I came up with to change the operation of the stock switch. You just need a simple 5 pin relay, the type commonly used for fog lights, electric fans or whatever. Here's my diagram below By "opening" one of the wires between the HEI pickup coil and the control module it will drop ignition pulses just like grounding out the points. A set of points within the OD solenoid will drop power to the relay and reconnect the ignition immediately upon the release of the OD solenoid, even if the throttle remains depressed and the kickdown switch, "switched", just as it did with the points.

Beware, if power is cut to the OD solenoid holding coil and engine load is not cut by killing the ignition at the same time, the OD will not dissengage properly. It will eventually dissengage when the engine/drive train load decreases to the necessary level. It will dissengage with a BANG and JOLT potentially damaging things.
Simply adjust the kickdown switch BEYOND operational limits, so the throttle linkage doesn't hit it. It won't drop out of OD when you floor it, but how often do you floor it anyway?