What is your objective? Do you want to run 87 octane or are you prepared to run higher octane. Was the block decked to establish the correct quench band. This will determine how much static compression the engine will tolerate without detonating. Un decked with a K cam - anything in the order of 10:1 will likely require 92 octane fuel. Decked with a correct quench band and you might do 10.5:1 on 87 octane - or so I have heard people claim.

Measure the clearance volume of the combustion chamber. With that value and your swept volume value you can play with various gasket thicknesses to calculate different compression ratios. There are various CR calculators available on the Web. Cometic makes B20 gaskets in various thicknesses. I don't know whether they make B 18 gaskets. If they don't, then you are constrained to using the original gaskets and may be limited in your CR options.