It looks like you have 3 options here, Roger. Order up a spring height measuring tool like Dwayne's, stuff some rope through the spark plug hole, pop off the retainer and measure the height accurately, take the heads off and take them to a shop, or continue to guestimate and roll the dice.

I would also put the windage tray back in your motor. I had to modify mine slightly to fit, but the Milidon tray does fit with the Road Race pan. Mine is off now, if you want to match the cuts.