As others have stated, you may be able to cut a case down to the legal length but if it were I and I was determined to use that rifle and also keep out of hot water i might cut down a few .33 Winchester cartridge cases to the 1.8" length and I am taking your word that you mean "Case length, not Over All Cartridge length, COL"
If i were to be satisfied with a cartridge I built with a cut down case and i am using a .33 Win as that case might work and not have .45-70 headstamp.
and accuracy is exceptable witch is a long shot with the long chamber and the way it tapers, anyway, say i like how this cartridge preforms, I would then get out a set of metal stamps and stamp this rifle with the name of this cartridge, like say, "45 Porkchop" and then I would carry with the ammunition a recipe card with the case dimensions on it. Now i certainly wouldn't have a box or even a single cartridge or cartridge case for a 45-70 in my vehicle or on my person when using this combination hunting.
I believe the Warden will judge your gun by the markings on it.