If I am understanding the question correctly, then it all depends on what timing you are starting at... So lets say you start at 13.5 BTDC. If I have my values correct, that should put the piston .063" below deck height assuming 4.724 stroke, 7.559 rod, 2.817 piston pin height, and 12.716 deck height. You are wanting to retard the timing to account for an extra .020" over head gasket so you want the piston to be .020" higher in the bore before injection? So that would mean you would need to set timing at 11.803 BTDC.