LonesomeP,
I had a similar situation last year. I had 44/40's loaded with smokeless for pistols using the original Winchester bullet mold. These were all short at about OAL of 1.585", using a Lee factory crimp. Then last year, I found a Rossi 92 in 44/40 and started to test the functioning with these same OAL bullets. As you can imagine the spring pressure in the rifle caused set-back of the bullets. The same thing happened in my Uberti 1866.
This then started me on experimenting to determine max length and crimping capabilities. The results were that a pistol and the Rossi would handle something over an OAL of 1.605" to 1.61". But my Uberti 1866 toggle-link did not like that 1.605" length and would not lever. I did determine that a max length for the 1866 was 1.604" and less was better.
In the end, I ended up using a RCBS 44-200-FN bullet mold that I had for years. That mold has a crimp groove and produces a final OAL of 1.582", which is very close to what a Winchester tong reloading tool and the original Winchester bullet mold produces as a final OAL for black powder loads.
You will just have to try different OAL in your new Winchester 1873 until they cycle. Remember with an 1873 lifter, that long bullet will not go up. Go REAL SLOW with this. It will hit the receiver and stop. Just push it down with you finger. You can push the cartridge forward, back into the magazine slightly then slowly work the cartridge back out the opened loading gate. Use something brass to push the cartridge. Or use Bunk's approach. Just do this until you get your maximum working length.
Hope this helps you out.