Welcome to the dark side!
I wish I could be there to see your smile the first time you shoot it!
The answer to you question on .454 or .457 etc. is....
It depends! I would buy both, and see what your gun likes. No two are the same so you will have to work up a load combination of ball and powder that your gun likes. Even two of the same guns from the same maker might like different loads, so, experiment with, it. Such is the joy of a BP shooter.
On a more practical note, when you seat the ball, it should shave a small ring of lead off the ball, which you will see as you retract the loading lever. If the ball seats with little or no pressure, it is too small. If it requires a Herculean effort to seat, it is too big. Go for Goldilocks, not too tight, not too loose.
Most BP revolvers will shoot most accurately at a load that is in between the minimum recommended and the hottest, or most that will fit in the chamber. Your Walker will fit an enormous powder charge. Probably won't be accurate at the highest powder charge, so somewhat less than that will give you good groups. I don't have a Walker, but do have a dragoon and it likes about 30 gr or so of 2F.
You also may find that the loading lever falls down every time you shoot it. This is common on Walkers. Many folks tie the loading lever to the barrel with a thin piece of leather after loading to prevent this. If the loading lever falls, then the loading ram blocks the bottom chamber preventing you from cycling to the next round. It is not just unique to reproductions.
You're gonna love it! My Dragoon (which is the son of a Walker) is my favorite to shoot.
PS. I assumed that this was your first cap and ball revolver. If so, read the post at the top of this forum "so you want to shoot cap and ball revolver".
There is a mountain of good advice there.
