During the last rebuild on mine, involving leveling the block, and shaving the head somewhat, to prevent out-of-normal-range compression ratios that would result from the reduction in volume of the combustion chamber, a `spacer gasket`was used, basically a steel plate with gasket on each side to compensate for the material lost during the block head machining.
Not sure if this is your issue, maybe using higher octane gas should help (I think), but at a certain point the increased compression ratio will be difficult to live with...

Don `87 Mitsu 2dr, Rubicon survivor, GModified.