Beveldown - quite a change from a flintlock.

I'd think a 270gr. to 300gr. might be better from the accuracy standpoint for the slow rifling. It is possible the 350gr. will shoot, though.

If the patched bullets will fit into the fired cases, fine and dandy. Merely shoot them to find out if the rifle likes them.

I used to rub softened 60% beeswax, 40% Vaseline into the paper before pushing the patched bullets into the cases. The lube helps hold them there if they are barely an interferance fit (which is perfect). Would have to see if recoil of the right barrel, allowed the left bullet to move. Partical sizing can be done with a backed out .45/70 (or other .45 rifle) die, only running the case part way up into the die. Thus, any tension desired, can be adjusted for.