I had an Enfield rifle converted to .45 and without telling me the gunsmith took it upon himself to thread the barrel .578x28 as opposed to the thread pitch I asked for.  The muzzle brake for my suppressor does not come in this pitch, which is why I didnt ask for it. Would a competent machinist be able to take the suppressor adapter I have in 5/8x24 and thread a sleeve (in both .578 (inner) and 5/8 (outer)) to go inbetween the correct muzzle break and the barrel?

Im not sure I am articlating this clear enough to make sense, Ill try and draw a picture when I get to a desktop.

Im trying to deal with the original gunsmith but I am not sending it back to him, I would rather use someone that I know can do the work correctly.

Thread minor is like .586 on a 5/8-24, so I'd say there's not enough meat left in there.

You could go to something non-standard on the barrel's threads to get it small enough to get some room in there as the machining could just make matching threads for the adapter.

Or you could go with a slight extension.

You've basically got two options IMO (this is assuming you don't have enough barrel length to just cut off the existing threads and start over), the simplest being to purchase or commission a step down type adapter (even though you're not actually stepping down in size, as mentioned you wouldn't have enough material to do a bushing type adapter) to convert your threads to the pattern you need. The other option will involve having more machine work done on your barrel, but would be the most "elegant" fix. You have the threads turned down to remove them (minor dia is around .546 ish iirc) then make a press fit bushing made to bring the barrel back to original barrel OD. It then can be TIG welded in place, cleaned up and re-threaded to your specified pattern.

This would be the best bet, I do this frequently on AK's, stepping up from 14 x 1 LH to 5/8 x 24. I will turn the original threads down, fab a snug press fit bushing and silver solder it in place. Indicate the bore and turn/thread as normal.