what gear ratio?
you can change to a thinner head gasket but that will not help much.
To make a differenece, either change to a smaller combustion chamber head or get some pistons with a small dome. 820's heads have 110cc chambers (10cc less than the 781's).
Or, use the 8:1 cr to your advantage and add some force induction (turbos, super charger, blower, or n02).