REbuild advice on parts

I've also been told specifically 91 octane in SoCal is lackluster...a very good friend of mine had his Mustang tuned by Mike Murrillo in San Antonio for 91 octane gas; ran great there and in Abilene...ran 12.40s all day long N/A 306. As soon as he PCSd to SoCal, he had nothing but problems and detonation, couldn't even run it at the strip there without blending it 50/50 or more with Sunoco or VP 94.