Going from 9.5 to 9.1 Compression 408 Magnum

Picking up a motor tomorrow that was built for 9.5:1 compression. I am planning on throwing my RT heads on but am hoping I can get the compression low enough to run 87 octane. Going into an older pickup used to tow a boat. I still have to get the motor and run the unknown numbers I don't have available at the moment (piston to deck height, piston dish cc....) in the meantime these are options I am considering to get me closer to to my target CR.

Gaining some cc's unshrouding valves on the heads
Thicker head gasket (cometic goes to .125)
Going to a deeper dish piston (currents are 25cc)
or having the piston relieved if possible.

The easiest might be running premium after all.......

Concerns
Grinding on the combustion chamber of the head
Surface roughness too much on the block for a cometic
Cost/availability of pistons
Weakening the pistons by removing whats needed

Looking forward to hearing responses!

Thanks!