Just completed a build (1st one!!) on the same bike. The inner diameter of the rear sprocket needed to be widened out with the handy dremel before mounting, to get it to fit over the coaster brake dust cap, but gave me no other troubles. In my experience even when your sprocket is mounted flush, true, and center, you'll still have problems with the chain rubbing your fender. What I did war remove the fender and cut it just below the top mount with a grinder and (if you're lucky and your misses has nail-polish in every color like mine) touch up the bare metal with something similarly colored to help aesthetics and prevent rust. This way you can still keep (most of) your rear fender and bye-bye loud, annoying chain rub.
Also, not trying to hijack the thread or anything, but does anyone happen to know the diameter of the rear hub?? Or which Manic Mechanics sprocket adapter fits it? I'm trying to ditch the rag joint but can't find this vital info anywhere. Thanks
-Troy