Re: Rag Joint
There is a lot of bad information that comes in some kit instructions. Mounting the sprocket against the spokes is one of them. Use both rubbers, sandwiching the spokes between them and take your time to get the sprocket wobble free. You'll need to put a slight 'Z' bend in the coaster brake arm to clear the sprocket bolts and you might need to either open up the center hole in the sprocket or grind off the lip of the dust cap over the left side bearings.
As far as the locknuts and lock washers; yes, use them both. You'll just have added asurance against the bolts loosening.
The sprocket adapters currently available are very good products and make sprocket installation a snap if you have the bucks for them. The rag joints get a bad rap from people but I maintain that if installed correctly will give you many miles of troublefree riding.
Good luck. I hope this gives you some direction.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"