Welding is a bad idea. Many reasons, so skipping that. Drill and tap the top hat for 3 6-32 allen screws 120 degrees apart. Take out flyweights and springs. I assume you know about the c-clip below the felt oil disk. The 3 allens carefully adjusted are used to avoid run out. There is a few 0.001" slop in the 5/16" to top hat ID, just one screw results in significant run out.