Rolling and welding is the usual way to go, same as the wheel rims, someone like Ashby will do them and are traction engine friendly.
Other option is a thick wall tube and bore it out which can sometimes be taken advantage of by doing away with the front door ring and and adaptor ring onto the boiler. M-Machine have 193.7 x 12.5 wall.
193.7 x 6 is a structural size so you may be able to get an offcut of that, and adjust other parts to suit
Edited By JasonB on 28/06/2021 13:28:51