WF Ranger District is in charge of the Selway, and they can provide info on the road as the date approaches. 1406-821-3269 is the number I have in my phone for them. Hopefully it's open. End of May is always right on the cusp. Look online for the flow converter that will allow you to translate the cfs on the gauge at lowell to feet at paradise. Almost everyone refers to Selway trips in feet at the Paradise gauge. Moose Creek down is where the majority of the big rapids that you hear about are located, so obviously there will be plenty of beta online about those drop. Don't, however, sleep on the upper section that gets pretty big and continuous at higher flows. I don't know the exact mileage, but it's a long section (similar to the upper MF Salmon at high water) that culminates at Holy Smokes. Be on your toes and go for it. Don't listen to the namby pambys who tell you to back out if the gauge is above 6 feet. It's all manageable for a capable boatman.
I've done a few high water Selways, so I can maybe answer some questions for you, but I was so drunk most of the time that it just blurs together.
Keep it Steep