You would be further ahead financially to sell this rifle and buy a purpose made .280 sporter like a Rem 700, Win M70 or Ruger M77.
Sporterizing an 03-A3 involves a lot of costs which would be well in excess of what you would pay for a purpose made sporting rifle already chambered for the .280. These would include purchasing and having a custom made barrel installed, metal refinishing, cost of a sporting stock, low scope safety and a custom trigger guard with hinged floorplate, alteration of the bolt handle and having the receiver drilled and tapped for a scope mount.