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Thanks.
The racks center tube is slightly thicker wall than the rest. I jammed a 6mm Allen key top into the end of it just enough that I could mount a M6 bolt onto it. The I welded the bolt and the tube together leaving a little gap so I was sure to penetrate to the Allen key top. That was just quicker than machining a threaded part and silver solder it into the tube like I normally do. I am certain it will be strong enough. It was a good tight fit.
Gearing is 44T / 12-28T. Copenhagen is absolutely flat and my mate is a former Bike Messenger. I reckon he'd be angry if gearing was too light. It is also a parts bin build and I don't have a longer rear mech.
Racks done