Howdy
A couple of things.
If you are looking for lever guns chambered for 44 Special, you may have a hard time finding one. Although Uberti does offer the chambering in the Model 1873, they are very difficult to come by. One would have to be special ordered, and they are not usually in stock. The two most common, and easily available chamberings for the 1860 Henry, 1866, and 1873 are 45 Colt and 44-40. Because of its immense popularity with the Cowboy crowd, ALL of these rifles that I have seen on gun store shelves have been chambered in 45 Colt.
Although Uberti has been talking about releasing the 1873 in 44 Mag for some time, I would be very leery about shooting 44 Mag in any toggle link gun.
Another thing to consider is the strength of the actions. The toggle link guns, specifically the Henry, Model 1866, and Model 1873 are notoriously weak. They will take SAMMI spec ammo, but if you want to get maximum horse power you are better off looking at the Model 1892 or the Marilin Model 1894. Both of these rifles are much stronger than the toggle link guns. That is why they are offered in 44 Mag. The '92 is even available in 454 Casull if you want maximum power.
Regarding Black Powder in lever guns. The 44-40 is the cat's meow in a rifle. Yes, the thin case wall makes it a bit fussy to reload, care needs to be taken in setting up the dies just right and one has to pay close attention, but that same thin case wall makes it obdurate under the relatively low pressure of Black Powder to seal the case far better than the thicker, more robust case of the 45 Colt. I shoot 44-40 with Black Powder in no less than five rifles chambered for 44-40. Clean up is a snap, all the fouling stays in the bore where it belongs. The same cannot be said about 45 Colt and Black Powder in a lever gun . Fouling will blow by the case and get into the action where it needs to be removed. Not a problem with the Model 1873, with its easily removable side plates, more problematic with the 'Henry or '66. This can be a real pain with a '92, they are not so easy to take down. Not a problem with the Marlin, one screw and the entire action comes apart. Unfortunately, all of the Marlins coming out of Ilion recently have been crap.
If you are talking Black Powder, the 44-40 has significantly more case capacity than 44 Special, and you can stuff more powder in, getting more oomph. Not so with Smokeless, where you don't fill the case up anyway.
Regarding rifling groove diameters of 44-40. Yes, in the 19th Century the standard was .427. However, many manufacturers today opt to use the same barrels that they do with 44 Mag/Special with .429 barrel groove dimensions. So in that case, the same bullets used for 44 Mag/Special can be used for 44-40. You really have to slug the barrels to be sure. Three of my 44-40 rifles have .427 barrels, 2 of them, including my Uberti Henry I bought brand spanky new a few years ago have .429 barrels. Most of what I have read about recent Uberti 44-40 rifles seems to indicate they are using .429 barrels these days.
44-40 revolvers are a whole nother kettle of fish. Some manufacturers use .429 barrels, some use .427. But many have had difficulty matching chamber mouth diameters to barrel groove diameters.
While I am still thinking of Black Powder, you are aware I hope that it performs much better with bullets specially lubed for it? Regular Smokeless lubed bullets can cause problems when used with Black Powder.
A few other things.
Planning on making your own Black Powder? I wouldn't.
Avoid the Winchester Model 1894 chambered for 45 Colt or any other short cartridge like the plague.
According to Kuhnhausen, 44 Special was first chambered in the Single Action Army in 1913, not 1940.
Neither 38 Special nor 357 Mag loaded with BP are slouches.
Yes, the brass frames of the 1860 Henry and 1866 Winchesters limit pressure, that is why they are not chambered for 357 Mag.
That's all for now, gotta go to work.
P.S.
Loading more than one caliber really is not a big deal.
All my Cowboy rifles are chambered for 44-40, most of my Cowboy revolvers are chambered for 45 Colt. After a while, you learn to tell them apart on sight.