The 405 flatnose and the 500 roundnose are what the rifle shot originally. Frank is right slug it the barrels have a shocking range of bore. Once you know what fits cast them in 20:1 that was the temper of what the army provided.

40 Rod

If the load you come up with shoots too high at 100 (bet it will!) the front sight blade can be replaced with a Krag or Springfield one. Pop the pin, slide the old blade out and the new one in and replace the pin. The arsenal was loath to change anything they didn't have to. The buttplate screws on the 1903 are the same as the 1863 musket.

The trapdoor I am buying from my friend's estate is in the same condition as yours. I had worked up loads for it for his wife to shoot. At the time custom cast bullets were available in 300 and 350 grain weights sized .460. The 300s would shoot into about 4.5 inches at 100 with 27 to 30 grains of 5744. The 350s would shoot into less than 3 inches (3 with 4 of 5 in 1.5) with 28 grains of 5744. Now that it will be my rifle I started looking at all my 45 moulds. The only one I have that will get to .460 is an Ohaus version of the 405 Govt. I have had good luck shooting the Lee 405 hollow base in another 45-70 that has a Martini-Henry barrel, also oversized (probably more than the Trapdoor), using 12 grains of Unique.