Flat top piston is best , dome impedes flame travel. The problem with 383's is piston selection sucks. There is no piston out there that has the same compression height as a stock 383HP piston and the factory lied about the compression ratio.
There is a member that has a piston available that is close to stock , you want one wit ha small valve relief , 3cc would be best .
The best piston would be a custom Diamond with a step head dome for use with the 906.
What is your budget ?
Just reread your post , you are in Cali , land of crap gas , you need to build it right so it will run on the swill that is at the pump.