The t-3 t-4 actuator should work just fine on your t-3. If it's over boosting the actuator might well be the problem.

Have you ever looked inside a internally gated turbo?

If not it has a hole in the exhaust housing that has a flapper that closes it. The idea of the waste gate actuator is that it pulls the puck forward and seals it from leaking boost but when pressure is applied it opens and forces the puck to open. That allows the hole in the hot side housing to leak out exhaust. The exhaust that leaks out never hits the wheels and doesnt allow it to boost further.
So if the rod falls off it doesn't pull the flapper closed. Then the hole is open and ever at wot and 6k rpm the turbo will have problems building more then a few psi.