You are going to pay someone how much money to rebarrel a perfectly good collectable rifle so that you can save money on buying another type of cartridge ?
I should stay out of conversations such as this because I always get myself into trouble, but: I am guessing you don't own the rifle since you mention that it can be EITHER a 1903 or a 1903A3 ?
So, if you get the rifle from the CMP thats $400, then pay ?? I am looking at a catalog right now and a guess might be around $400 for the barrel work (barrel (decent-not great barrel), and installation not counting any kind of finish).
This doesn't sound like a good deal to me.
However, I have seen 1903 actions for sale in shotgun news and the CMP might even have some. So, you might drop the cost by $100-$200, so you would only have $600-$800 tied up in this venture.
But, if that is your dream rifle, why not ?