what compression is needed for 91 octane

So I am living in San Diego and the highest octane at the gas stations is 91 my 73 340 has 10 to 1 compresstion I have been mixing race gas but I am really tired of paying 8.99 for a gallon of it I looking into maybe doing some head work to reduce this need for mixing gas . what compression ratio would I need to safely run 91 octane ?
Running out of the box air gap intake and rpm preformer eddy heads with a Hughes whiplash cam