If you do fit a new No5 flash eliminator to the barrel and the taper holes don't align, just file a small flat at the top or bottom bearing surface of the barrel, where the flash eliminator fits, soft solder or tin the mating surfaces of both, heat up and slide on. THEN align to UPRIGHT on a sheet of glass.
It should stay firm forever. I did this when I rebarrelled a friends No5 using a 7.62mm Bren L4 barrel 30 or so years ago. And it's still going strong. In fact, he's using it today!