Re: 5/8-24 thread on Rem 700 youth barrel?
Probably not, usually something around at least 3/4" (.750) or near that is what you would want.
While you might be able to clean up the threads at that OD if the bore was concentric, there would be no external shoulder for the accessory to line up on.
I would have the rifle threaded 1/2-28 and have a 1/2-28 to 5/8-24 thread adapter (semi) permanently attached. This works pretty well.