no third gear

Here is my 2 cents, I burned up a tranny years ago thinking the linkage was just for kick-down purposes. 16 at the time. Assuming the trans worked properly before the rebuild, verify the lever on the trans moves in accordance with movement of the linkage at the carb. In theory, when the carb is wide open, the level should be all the way back, you fine tune from there. The spring should hold the carb linkage pin to the back of the slot, meaning as soon as the butterfly opens, you should be moving the linkage and ultimately the lever on the trans at the same time. I found when mine was not connected properly, I thought I had no 3rd gear, but in actuality, i think it was going from 1st to 3rd very quickly, resulting in the mushy feel. Sounds to me like your not getting enough input to the trans from the carb linkage being open. Curious if this helps or i'm wasting space.