I used to load the old Dominion balloon-head .455 Colt (MarkI) cases using as-cast 230 grain RN bullets designed for .45 ACP.
(Lyman #452374 mold using reclaimed .22LR lead or wheel weights measuring about .454")
Since the case volume seemed similar, I used a 5.9 grain dipper of Hercules Unique from my Lee Loader, for a pressure of around 12,000 CUP for the .45 ACP. (Probably less in the .455 Colt case due to greater case volume.)
I fired these loads in a Colt New Service as well as in a Webley Mark IV .455.
In retrospect, since the Webley was an 1890s revolver, 5.0 grains of Unique would probably have been safer, though I experienced no signs of stress, and the accuracy was very good.
Since your example is an antique, I suppose that it might be a Mark IV like mine, or even a Mark I or Mark II or III)
The Mark I, Mark II, and Mark III would probably be less forgiving, being possibly black powder vintage guns.
As such, perhaps 4.0 grains of Unique would be prudent.
Rather than convert the gun, it would be better to make .455 cases from .45 Long Colt cases.
In addition to shortening to .455 Mark I cartridge length, you would have to turn the case rims down from the inside, as the.455 rims are thinner.
You can thin the rims down first in a wood lathe with a chuck, a drill press, or even a 1/2" drill clamped in a vise.
Wrap the cases with masking tape and use a file with a safe smooth edge to thin the rims a bit, using the revolver itself as a go-no go gage, then perhaps a dial vernier caliper. (easier than it might sound) (.005" taken off might be enough to allow free rotation.)
Then cut the cases down with a jeweler's saw to rough overall length. Then use a bench-mounted case trimmer to reach .855" overall case length. (Or just carefully file them to final length with a fine smooth file.)
Then chamfer the case mouths.
You can load these cases using .45 ACP dies with a .45 LC shell holder, or even with a .45 ACP Lee Classic Loader.