I made a clamp and pusher block after you posted it some time ago and it works a treat. I have noticed that the 5mm thick rubber seal sold by Drags, which fits in a metal 'top hat', is very hard and cannot be compressed to any significant degree unlike the old felt washer. The same washer/top hat seal is usually supplied by tony cooper when he recons a Mag. The 'top hat' fits into the recess on the back of the timing case. The 5mm seal thickness appears to set the mag at the correct offset so the mag and Cam sprockets line up. Being 'solid' it makes getting the Magneto square to the timing case a lot easier than with the felt washer.
Regarding the chain and power strokes I noted that, when the engine was turned by one revolution, the chain ended up in three different positions, relative to the sprockets, as the engine was rotated each revolution. i.e it took 3 revolutions to get the chain back to the same position on the sprockets. So in theory that should even out any chain stretch.