Check this out: flathead ford mechanics in california - Search (bing.com) Hand H is the big name you hear the most about. Better have deep pockets.
Probably guys that have no advertising and a waiting list. I hope you get some good references, but asking for a superlative is a clickbait ***le type of thing. There are 39.5 million people in California. A one-in-a-million person would amount to 40 people and you want the top 5. The area is 163,696 square miles. You can see why using "Best in CA" is hyperbole. Oh, and it's who, not what. Sorry, I'm grumpy tonight.
We had one that came out to homes and shops to do valve seat grinding. The valves, etc, he took home to spiff them up and came back to do the ***embly. There was a group of old guys that had a shop, nothing but old Mopars. They could do anything, really like stepping back to 1950-something when I visited them. The house, yard and garage are long gone, as the old guys too.
@bagel56 I see you had a problem with a recent rebuild. I am guessing you are not getting anywhere with the builder. Tear it down, find out what is wrong, and rebuild it yourself. They're not too difficult. We can help
Johnson Automotive Machine in Yucaipa. 909-797-2063. I haven't used the personally, but a lot of my friends have and are happy with them.
BMS machine in San Bernardino did the 8ba short block I have in my '39 p/u. Very happy with Dave's work. (909) 799-7770 1904 Riverview Dr # 101, San Bernardino, CA 92408
Johnson Automotive was also on the list for a good flathead machinist as well as BMS machine. Both are small shops and are very busy so don't be in a hurry. But either are a good choice. IMO
That's good for thought! Can either shop do an engine diagnosis? I 1st want to take the motor to the original builder for an engine diagnosis, even though he's in Fresno California.