Your first mistake was to change the head gaskets.....you stated you did it as a precaution, so apparently they were not leaking / blown, so no real need to be changed
As for the washers, pull one bolt at a time, put the washer on it correctly, snug it up but not torque it, when all the washers have been installed, then torque them up in proper sequence.....yes you have to lay on the engine, it's uncomfortable, pull the air cleaner stud out of the carb before you impail yourself
