Nice story, but we miss a few details: What sort of damage was on the old thread - what sort of tap, oversize or just cleaning up ? Oversize at what amount - and if so, I´d rather not want to do the job, even less so on the rear head on a twin on the bike. Thinking about all these fu***d up threads just by dodgy mounting the ex pipes and nuts by less talented owners a tap is a bad idea to give to them. And having an alu tube welded there for a bigger ex pipe to go over it is not a lasting repair, thinking of post war Horexes with alu heads. They had exactly this over-stub design. In real life the alu gets quite soft with all that heat and with a steel pipe and clamp it just collapses in time for a wiggly ex system, unless you accept this and have a few more positions to fix the pipes and all thoroughly to the bike so vibrations plus heat growth in cycles will eat less into the alu stubs. Not the best idea for long lasting joy.
Vic