When I had an ST300 without a kickdown switch, I made one for uner $10.00 and it worked well.

I bought a microswitch from an electronics store and soldered two leads to the normally open positions on the switch. One lead went to the black wire with yellow stripe providing power to the windshield washer. The other lead went to the downshift solenoid on the side of the tranny.



I used a piece of scrap aluminum to make a bracket to hold the switch with the left rear carburetor hold down bolt on my Quadrajet. I positioned the switch so that it was open until the throttle was floored.



When the throttle was wide open, the switch was thrown by the mechanical part of the secondary linkage on the left side of the quadrajet.