correct, if not done properly, it will either 'senget' or vibrate.
detail measurement is a must, to make sure it stay as close as possible to the windscreen and the steering column face the driver. can't really explain how easy/hard it is, but if your mech has not done this before, then it will be 50/50 for him and you. it will depend on how much you wanna trust his skill and creativity. unless you want greater assurance from someone who have been there/done that, then why dun you look for other mech. there are many reputable toyota mech in KL, such as trial tuning in sg.besi, powerspeed in kajang, Danny autoworks cheras, kenny puchong, Goodspeed kepong..juz to name a few.these few mech can pasang the dashboard with close one eye.