Is that just a thread on fitting over the other threads?I went a similar route to thread a tikka t3x sporter barrel to 5/8x24. My barrel started as a 7 rem mag but was bored out to 338 win mag. I shipped it to ADCO Firearms for them to chop, thread, and install their tapered shoulder. Total cost was about $175 with shipping from Anchorage and had it back in less than two weeks.
The tapered shoulder came loose while fiddling with it when I got it but I retightened it with some blue loctite. It's been super solid, and accuracy and repeatability has been great.
It'll be nice when sporter barrels all come with belled and threaded muzzles.