a few things to watch out for
1) make sure the pushrod meets the actuator arm at 90 deg. this helps keep the force vs distance curve monotonic, which in turn makes tuning easier
2) before you take the old actuator off, get hand pump with gauge and pressurize the old unit until it just starts to move. take note of the pressure. when you install the new unit, you can then adjust the preload such that the same pressure also just starts to move the actuator. this will save you tuning time. the same length for two different actuators may not result in the same preload. both manufacturing tolerances and aging of the old unit are working against you.
3) make sure the pushrod is not rubbing anything. this includes the edge of the actuator itself.
4) while the old actuator is off, check that the wastegate operates smoothly.