455 Ammo, Cases; Rebore to 45 Long Colt?
1. Fiocchi after a long hiatus has started importing the 455 Webley Mk II load again. Your best bet is to buy coupla hundred rounds of that and reload as needed. I bought the 455 empty brass from Midway two years ago and tried reloading it but it was dimensionally "off" a bit, I can't remember exactly how (primer pocket needed reaming? rim tad too thick?), but I did put it on my Don't Buy Again list. Reloading is easy just remember the 455 is a lower pressure round than the ACP.
2. The other thing you can do is have a gunsmith rebore (reamers) the cylinder to 45 Long Colt. This requires more skill than I suspect the average Joe has but it was fairly common in the post-war era, you'd get these firearms either converted to LC and so marked by the Brits or the Canadians or an importer would do it. Compare a LC ctg to a 455 ctg and see how similar they are. N.B. I don't have access to my library right now so this is from memory, any other poster please correct as needed. The 45 LC is .454 while the .455 is .454 to .455 true diameter, so it is a much better 'Match' than the ACP. The ACP only runs 451 so if you simply shave off the rear cylinder plate to allow for the thicker rim of the ACP you will eventually wear out the bore from excess gas escaping around the undersized 451 bullet. Not to say it isn't done and it works well but reaming to LC is better for accuracy.
3. I say again, remember that the 455 was designed as a lower pressure load than the ACP. Don't try to replicate Cor Bon ballistics in an old warhorse!
Good luck.