The variable pitch feature is controlled electronically by a solenoid in the transmission. When 12V is applied, the vanes in the torque converter shift angles giving it a different stall speed. On the OEM equipped cars the converter went into high stall on two ocassions: First - when the car was at a complete stop; this avoided creeping and was controlled by a switch on the brake pedal. Second - when the driver got heavy with the right foot and put the car into passing gear. The gears shifted as the converter went into high stall.
You're correct; the use of the ST300 converter behind the 401/425 will give it an even higher stall.
There are two terminals on the outside of the transmission, one for the kickdown, and one for the torque converter. All GM TH400's have electric kickdowns; you can borrow from one of them, use the OEM piece that came with the car (requires carb, linkage, and brackets from the same year), or there are aftermarket pieces available from Jegs and Summit.
I tried attaching an illustration of how to put a toggle switch into the OEM setup, but the file is too big. Give me your email address and I'll send it to you directly.
Ed
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