I'm not sure what the stock length is but call the Harley dealer in Pensacola and they can tell you.
I'm not sure you need to go to a longer than stock shock, you can get the heavy duting Progressives in about any model. I have some heavy duty 440's and I ride two up with approx 360 lb load and I don't need to make any preload adj to the shock....no problem with "bottom out".