It could be done, but as has been pointed out you'd have to wrap paper or wire around the .45 Long Colt case to keep it from moving forward in the .45-70 chamber, out of reach of the firing pin.
I had a book years ago about single-shot rifles. In it, the author mentioned that if you took a chamber cast of a .45-70 Springfield to get measurements, you could make an adaptor from steel that would slip into the chamber and allow you to shoot .45 Long Colt ammo.
But, he stressed, a chamber cast was required to get as much "meat" in the adaptor as possible. Toward the end of the .45-70 chamber, it was mighty thin.
The .45-70 will fire 2-1/2" .410 shotshells but it's not recommended with plastic cases. An old, late friend who had been raised in Bemidji, Minnesota in the 1930s and 1940s recalled his father carrying a few of the shorter .410 shells with a Springfield while hunting deer, in case they saw a grouse at close range.
So, I tried it one day -- with vintage .410 shells made of paper. The paper cartridges tended to swell and block the backflow of rushing gas much better than the modern, plastic shells. The plastic shells split their cases and brass bases. I tried this in both my reproduction H&R 1873 Springfield, and my 1977-made Marlin 1895. Same result in both rifles.
When I hunted elk in northern Idaho (love the St. Joe River drainage!) with my Marlin 1895 I carried a few paper-cased .410 shells factory loaded with No. 6 shot. Never had a chance to bag a grouse, but it felt good having the option.
I never loaded the Marlin's magazine tube, just singly fed the .410s into the chamber. I suspect that loading the .410 in the tube would cause a jam.
No need to look for old, paper .410 shells of 2-1/2" length, though. You can make your own shotshells for the .45-70 if you use a .410 wad cup, some Unique powder, and a .45-caliber felt wad (most often used for cap and ball revolvers) under and over the filled wad. I prefer No. 6 shot. Crimp the mouth of the case over the front, seated wad to hold everything in place.
Works okay, but maximum effective range with old .410 shells or the homemade .45-70 shotshells is about 20 feet. Patterns open up too much beyond that for a sure kill.
In northern Idaho, before I purchased my Remington 870 20 gauge with a short barrel, my home defense gun was my .45-70 and my own reloads that used three .457" Speer balls in the case. One shot: three .45-caliber balls screaming at ya!
How I assembled such loads is another story entirely ...