I am going to measure my crank pulley and sc pulley to be sure what is on my SL55. What is best way to do this? Do I measure overall diameter? Bottom of groove diameter? Or is it so simple as, "they are already marked and I just have to read the markings?

I believe you could measure around the pulley(where the belt rides) and divide it by 3.14 then take those inches(if your in America) and convert it to MM's.