915 J heads, need specific machining details to squeeze as much as possible out of them

From what I gather, the 915 J heads aren't going to flow much different than the eddys if machined properly so I've gone ahead and bought a pair for $300. They are untouched from factory out of a 71 360. Im in Australia and there aren't any machine shops that are familiar with what I need to do to them now to get them to their 90% potential so I need specifics to tell my machinist exactly what to do to them. Hoping for experienced advice. Thanks for the help!