I need a switch i can bolt to the thorttle linkage of a quadrjet that is normally closed but will open when the throttle closes (to idle) - (break a circuit at idle, complete the circuit when off idle)

reason is i have a 7000R4 OD trans with a locking torque converter and no computer to lock it up. they sell kits for this (from $70 to $125) but I can achieve the same results IF i can find a way to break an electrical circuit when 'coasting'.

I can lock it up when in 4th easily enough, but need to break the circuit at idle (coasting) to unlock the converter - so i need a switch - any ideas?

Some of the 'kits' come with a vacuum switch to do this, and that might work just as well, but i figure such a switch would be harder to find. (normally closed, open circuit at high vacuum...possibly adjustable?)