I belive they are steel casting, and welding compatible without any special measure. Also welded to the central pivot, usually underneath the item.
When contemporary, a friend altered a 750 F2 example, by milling off original lugs, this to give a flat site to then weld in place significantly stronger and larger form items to make them more robust.
At the time, we were both working in engineering with metallurgy and analysis capabilities, checked through there as to material which verified the above view.
Plain stick method or decent grunt in mig etc to get competent amalgamation of weld site, else the result will just snap off easily.
If you want to build up a weld "lump" though, grind away flat, cover with lines of weld bead then clean the surface and go again on top. Repeating to height, you may need to grind top surface intermediately to get get good secure layering in building up.
Don't quench at all, just let cool between welds to avoid any crystalline risk.
We examined this process too during weld training, building up layers until sufficient to then cut the whole into cross section, ultimately to be then acid etched to expose any included faults etc.
