Re: Overcoming of the brake arm/sprocket clash?
Some hubs need the bearing dust cover under the coaster brake arm modified to fit in the center hole of the sprocket without rubbing when the ragjoint is tightened as well.
This can be done with a dremel or a grinder and it's a pain in the behind if it IS needed.
I tend to just remove it on my personal bikes since I grease my hubs regularly, but it's a bad idea on a sale build or if you don't take excellent care of your hubs and bearings.
If more is better, then too much is just enough.
If you can't afford it, build it yourself.