Just saw this post a month late, but I'll respond anyway for other who might be looking for a classic Mustang mechanic in the 818.
I take mine to Pat Law at Willard Automotive in Simi Valley, highly recommended by local Mustang club members. He knows his stuff: Auto Repair Shop, Oil Changes | Simi Valley, CA
For transmission & brakes I use Albert's Auto Transmissions: https://www.albertautotransmissions.com/
Other shops in the 818 that people have recommended to me:
Mustangs Etc in Van Nuys. I have seen several cars they have restored and they were top notch. https://www.mustangsetc.com/
Marlo Frame & Alignment Marlo's Frame & Alignment
Had a guy named Mustang Marty put a card on my car the other week at a car meet - Untitled
There is Mustang Service Center in North Hollywood, haven't heard much about this shop: Mustang Service Center