I cannot seem to find an adapter to fit it to 5/8-24, which is what all the .338 cans are. Should I have the barrel threads modified or have an adapter made? It currently has a Vais brake on it.
I would have it turned down and threaded for 5/8-24 and run with it. Minor dia for 3a threads is .642 and major dia for 5/8-24 is .625 so you should be gtg.
I would cut them off and do 3/4-24, cost will be same and you won't compromise with smaller than ideal threads on a 338 bore. Most 338 cans aren't 5/8 btw, they should be 3/4.