"What calibres would work well in a .410 double rifle conversion"

Don't get carried away with yourself, too much enthusiasm is not a good thing, when you are considering such issues. I would say that, for nitro cartridges, you should do as Ron Villa has done, and stick with .22 hornet, or anything else with similar, or less, rearward thrust to action, while allowing for chamber, barrel, diameter comparable to your .410. That means that .303 is out of the question. You would be well advised to first read W. Ellis Brown's book on converting double shotguns to double rifles.