It depends on the type of rifle you aspire to build. My mental picture of a .303 single shot has a long, slender barrel and short forend. If you want this type of rifle you would best begin with a Ruger 1A in any caliber. You get the forend channel that is correct, the barrel has a clunky front site and sling band you can use on the .303 barrel, and the 1/4 rib will also likely work. The only part you have to buy is an extractor for .405, about 15 bucks. The 405 #1 has a heavier barrel that really wouldn't let you use any of these parts on a slender 303 barrel.
Otto