Have the new barrel put on. If it feeds, just great and it probably will. If the gun fed correctly as a 7mm, it will probably feel perfectly as a .358NM. The Norma is a bit longer but the belt, the deciding factor in most feed problems, is the same, so feeding should be the same. There is no reason to add to the cost of a job for problems that don't exist. Any gun smith worth his salt will check feeding after re-barreling.
Yes - he needs ammo to check headspace and feeding properly.