D_MAN: the twist on a 7x57mm Mauser is 1:10. The twist on the .280 is 1:9.5. I don't know if this will affect the accuracy of the loads you want to put through that rechambered 7mm barrel but maybe you should run this thought past some of the other forums, let's say the Custom Rifle forum, to see if anyone else has had any successes or failures with this approach.
You may recall on of the threads we had here once about using heavy slow moving bullets in the 7.62x39 and turning it into a 300 Whisper type cartridge?
The Whisper cartridges were shot from barrels with one twist and the standard 7.62x39mm barrel has a twist that was one in ten or one half in ten different and opinions and experiences provided felt they may not work that well - accuracy wise. It may be the same with your 7mm/280 conversion. More research might be needed. HTH. Mikey.