Yeah gphil, I've always said the hardest part of the build is getting the chain alignment right. The best way I've found is to run yer chain over the engine sprocket, stretch it straight back while looking down the chain. You can see when it's center on the engine sprocket. STRAIGHT BACK from there is where your rear sprocket should be. You can try a straight edge but that never worked very well for me.
You can add or take away spacing on the sprocket to get it right. On some I've used two of those rubber spacers, on others I've cut them down to 1/2 thickness to get it closer to the hub.
I haven't used the rag joint mount on any of my personal bikes for a while now, just on customer bikes. A good hub adapter is the only way to go. I've got the Howard Sprocket Adapter on mine. But any decent adapter is better than the rag joint.
P.S. I'm 58 years young. Never say die.