Re: old 6.5/308 dies for .260 Rem?
The die set should work fine.
The sizing die should be a bushing die, being micrometer. If the guy is having a custom gun made, give him the dies, he can send the sizer and bump- body die to his smith and have the smith ream the dies to his chamber, polish them.
The gunsmith should have a good idea on what size bushing to run. The seater die should work flawlessly. Sounds like a win-win situation. Truly custom die!
If it is a factory gun on order through some gun dealer, or shop, disregard my advice.
Milo