Here is the fuel tank on my version of my just completed Rupnow vertical
The tank is all stainless steel 28mm dia tube 70mm long with a 2 mm wall
The ends are a light press fit with a smear of green loctite.
congratulations sir on getting your Webster engine running.
I had the same problem with the head rocking back and fourth I fixed it by putting two extra 4mm screws in the bottom, it worked for me.