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I want to (will) use a single speed Torpedo hub
I don't think I'd compromise my frame design in order to use something which is in every way imaginable inferior to a flip-flop hub. Then again, I wouldn't even use a flip-flop hub, as being able to swap from fixed to free is one of those things which seems like a good idea but turns out in most cases just to be a way of carrying around an unused sprocket or freewheel for years.
Let me guess - chainring swaged to the crank (no spider), gas-pipe tubing to retain stiffness despite the crimps, not strictly speaking street-legal clearance between tyre and tube? There are ways and means, but as I said, if you want proper clearance and the ability to use any track chainset, you're too short of space.