If your stock was born a 1903 rifle your 1903a3 rifle barreled action would never fit into this stock without cutting the forend ring groove. You can put a 1903 barreled action in a 1903a3 stock, but not the other way around if you intend to use the upper hand guard wood.
A 1903 will not fit in an A3 stock without relieving the stock for the rear sight collar.