Howdy
I would be leery of a Model 1873 chambered for 44 Mag. The '73, just like the '66 and the 1860 Henry are toggle link actions and they are not known for strength. 44 Mag is a very new chambering for the '73, only a couple of years now. Not a whole lot of mileage on them yet, and they are rare as hen's teeth anyway. As has already been stated, all firearms produced in Italy must be proofed in government run proof houses. That is the law. But proofing only involves firing one or two proof loads, producing about 1/3 more pressure than the SAAMI Max. Yes, the guns have been proofed and they did not blow up. That is all proofing accomplishes. Just how well a 44 Mag '73 will stand up to the pounding of many, many rounds over the years is a completely different story. Don't forget, all the toggle link rifles basically have skeletonized frames. There ain't a whole lot of solid metal there. And a toggle link simply does not lock up as securely as a more modern design.
I had a chance to examine a 44 Mag '73 a year or so ago. It was right next to a 45 Colt '73. As far as I could tell, there was nothing stronger about the 44 Mag. The frame walls seemed to be the same thickness, and the bolt was the same. No idea if the links were more robust, but I tend to doubt it. Frankly, the 44 Mag '73s have not been on the market very long and I have not seen any information at all about how well they are holding up.
I would not go that direction.
If I wanted a 44 Mag rifle, I would buy a Marlin Model 1894 or a knock off of the Winchester Model 1892. Not a Winchester Model 1894. The 1894 is a longer action, based on the 30-30 cartridge, and they have been modified to accept the 45 Colt cartridge. Everybody I know in CAS who had a 45 Colt 1894 eventually got rid of it. Too much jamming because it was designed for a longer cartridge.
Yes, the 44-40 cartridge in historically correct for both the SAA and the Winchester 1873. 44-40 was created for the Model 1873 in 1873. Five years later in 1878 Colt first chambered the SAA for 44-40. I have about five rifles chambered for 44-40, I love the cartridge. But beware, it can be a bit fussy to reload because of the extra thin brass at the case mouth.
For what it's worth, despite the fact that lever guns are now available in 45 Colt, that is not a historically accurate chambering for any rifle. Totally a modern development. If you want to be historically accurate and shoot the same cartridge in rifle as well as pistol, your choices are 44-40, 38-40, and 32-20. Historical accuracy ain't everything, but the 44-40 loaded with Black Powder is the perfect round for a rifle.
I love the 45 Colt. First round I learned to load. Nice big bullets and big cases, easy to handle. Buy a set of carbide dies and you won't be sorry. Not really a big investment, just a set of dies. I buy 45 Colt brass direct from Starline, usually 250 at a time. Lots of commercial cast bullets available for 45 Colt too. I do not own any rifles chambered for 45 Colt, but I have 2 Colt SAAs, 3 Rugers, a couple of clones, a couple of Remingtons with conversion cylinders, and this really cool old Colt New Service made in 1906 that I fired for the first time Saturday.
) as well as the .45 (albeit w/ smaller holes). I hadn't noticed the impact that "the judge" and others have made on the commercial .45 Colt ammo market, so maybe that has changed. 
