If the transmission is already flaring going into high gear, the damage is done. The 3rd gear clutches are damaged. You can install the throttle pressure linkage, or convert to a cable and it may improve, but the damage is done. High gear will be weak from here on out.
That linkage does not just perform the kickdown feature. It also controls the transmission's throttle pressure. The more you press the gas pedal asking for more power, the more line pressure the transmission needs to see in order to hold the clutch packs together under acceleration and not slip.
Removing that linkage, or not having it adjusted correctly will burn the transmission up one forward gear at a time, starting with high gear, then second, the finally first. Your third gear clutches are already damaged. Even shifting manually does not solve the problem, contrary to what some people say. It must have the throttle pressure linkage or cable and it must be adjusted correctly for the transmission to work correctly.
Go ahead and either get a cable setup or the factory linkage setup and adjust it correctly. It will likely improve it, but I am afraid high gear is damaged beyond hope at this point.
