The major diameter of 5/8-24-RH threads is .625", so you will need more than that to have any kind of a shoulder. 5spd's recommendation of .685" is probably a bare minimum IMHO. That's only .030" of material on each side to shoulder on. My personal preference is a bit more "meat" for the shoulder than that.

Depending on your caliber, you could get 9/16-24-RH threads done, or even 1/2-28-RH if its a .224" caliber. These are all just my observations and opinions, but I am basing them on my 10 years of manual and CNC machining experience, so take all that for what its worth.

Get ahold of Jim down at www.apachegunworks.com and ask him his thoughts and see what he would charge you to thread your barrels. He does great work and is a great guy to talk to.