Yes, I have this pulley on my Cafe' bike with the 340mm shocks, and it works fine. You do need to run the belt a bit looser with any of the longer shocks, as the increased swing arm angle at rest results in the belt getting tighter as the shocks compress. I recommend that you put the bike on a jack, remove the shocks, and then lower the bike until there is a straight line between the swing arm pivot point, the swing arm, and the rear axle. When all 3 of these are in a straight line - that is the point at which the belt will be the tightest. Adjust the belt tension at this point, then jack the bike up and put the shocks back on - then see how much play is in the belt.....and keep the belt adjusted at that amount of play thereafter.
I don't know exactly to tell you about gravel. I have ridden on gravel roads once or twice a year - but I don' make a habit of it. The front pulley is not likely to get any gravel in it, as the belt engages at the top. The rear pulley is most apt to get gravel, as the belt enters at the bottom. If a piece of gravel is small enough to get embedded into a groove in the rubber belt - it likely will pass through at the front pulley. There have been members that get gravel stuck in the pulley or the belt, and sometimes it will make a hole in the belt. Youzuguyz in Texas rides on gravel roads frequently, and he has the stock belt guards in place and he has never reported having frequent problems.
I was able to find another used pulley and it was delivered yesterday. Part of the process involves cleaning the pulley when it arrives, and getting these clean enough to powder coat does take a considerable amount of time and effort. The Kawasaki donor bikes evidently don't have oil seals that keep all the oil inside the engine! This is good news as the pulleys are never very rusty - but it is a chore to clean them up. It often includes a week soaking in gasoline, a scrubbing,pressure washing, cleaning in my ultrasonic cleanser, and a bit more scrubbing! It is time consuming to get the dirt out of the corners of all those teeth - the 25 teeth have 100 corners to clean!