Well I managed to get my head into gear and work out a way to get past my issue of not being able to retain the cylinders in the chassis without the body in place.
From a piece of 3mm diameter nickel rod I turned a short length down to 2.2mm and threaded it 8BA. I cut it off using a piercing saw then reversed it and face it off until I had just the merest hint of a head as the thread ends.
I drilled a couple of holes in the top plate between the cylinders and tapped them 8ba there was already a slot in the frame spacer to accommodate one of the screws but I had to drill a corresponding hole to allow the second screw to pass through the spacer.
There isn’t a huge amount of screw thread in the holes so I soldered the screws in to reinforce them.