.45 ACP is smaller diameter than the .45 colt and not only will it not head space (no rim), it'll drop right through a .45 colt chamber. The fact that it won't work will be blatantly obvious.
The .45ACP, like a lot of semi auto cartridges, head spaces on the lip front of the case. There is a little step inside the chamber for head spacing. It is rimless. .45 colts head space on the rim and there is no lip in the chambers.
There were some Blackhawks with dual cylinders available in the past that would shoot both rounds from the same gun, but they're not available now. Another difference in the past was that .45 colt barrels/bullets were .454". That hasn't been the case in the last 60 years, pretty much have standardized on .451" just like the ACP round.
I see no real reason to care to shoot .45ACP in a single action revolver. In a DA, the moon clips make for quick reloads, can be an advantage especially in competitions that require reloads. But, in a single action, I'd as soon just shoot the big Colt. I handload, so I can load 'em however I want. I have two .45 colt loads for my Blackhawk, one pushing a 255 grain flat nose at about 900, basic BP equivalent or a little more. The other pushes a 300 grain Horandy XTP JHP to 1200 fps out of a 4 5/8" barrel, a powerful load capable of bringing down some heavy game and in the .44 magnum range for power. The lighter load is more pleasant to shoot, obviously.
I really don't see why anyone would want to butcher a beautiful Old Army cap and ball to chamber it for .45 Colt. Just buy a blackhawk, heck-u-va gun there. You can get it with a 6 1/2 inch barrel if you want a long tube, long enough, or you can get it in 7 1/2 inch I believe in the Vaquero. My Blackhawk in .45 Colt is one of my favorite outdoor guns, especially if I'm hiking in bear country or hog hunting.
To me, the very reason for owning an Old Army is because it's a front loader. It's a danged accurate front loader and it makes a great back up for hunting game when you can only hunt with black powder, which I find cool. I've owned an Old Army for about 30 years and love to shoot the things. For hunting back up, I shoot a cylinder full of Pyrodex P behind the Lee 220 rain conical hollow point. It pushes around 900 fps, nothing to sneeze at! I had a stainless one that got ripped off and replaced it with a blued one. I'd like another stainless one someday as it was so easy to care for and clean up after shooting, just pop in the dish washer after pulling the cylinder.