Is the kickdown linkage set up right?
Ditch the bolt that you have installed to take up slack. Add an extension cut from all thread. Couple it together with all-thread coupling nut like this. The kickdown linkage has to move exactly at the same time as the throttle linkage. This is achieved by the throttle bolt/stem being pushed up against the yoke, like my picture. You have to add an extension made from all thread if your yoke doesn’t reach the stem. It’s incredibly easy to achieve.
You have too much slack at the adjustable yoke. The yoke needs to rest up against the carburetor throttle bolt/stem. Just like my picture. With that much slack, the tranny won’t even shift gears correctly. Let alone, the kickdown work properly. You will know, without a doubt. When it kicks into passing gear. Approximately between 45- 55mph, when you stomp on the pedal, to the floor. It will make a very distinctive sound. ( Wa-wump ) The engine makes this sound because the transmission is downshifting into 2nd gear at Drive gear road speeds. Causing the engine to rev higher than normal. This creates more torque than staying in Drive gear. More torque or more power to pass. Hence, why it’s called passing gear. Passing gear is actually 2nd gear. The transmission automatically drops down into 2nd. Rather than the operator dropping down into 2nd, manually. For this function to operate properly, it has to be adjusted properly. Once it’s adjusted properly and if it still doesn’t kick down. The transmission would need mildly rebuilt. It would need the valve body pistons polished or clean very lightly. Valve body pistons can slightly pit or become slightly scratched. This causes the valve body pistons to stick in position. This is caused from people not changing transmission fluid. This can be addressed without removing the transmission. Easy as easy gets. If that were the case for anyone. But, you haven’t gotten that far.
Ditch the bolt that you have installed to take up slack. Add an extension cut from all thread. Couple it together with all-thread coupling nut like this. The kickdown linkage has to move exactly at the same time as the throttle linkage. This is achieved by the throttle bolt/stem being pushed up against the yoke, like my picture. You have to add an extension made from all thread if your yoke doesn’t reach the stem. It’s incredibly easy to achieve.