I have never heard of anyone shooting 45-70 in a 45-90 chamber. A .22LR is shorter than a .22 mag but if you have ever seen it tried, you will know why not to do it.
Take a look at this posting.
http://castboolits.gunloads.com/I have shot a limited number of .38 Spl ammo in a .357 mag but I hate doing so as it really junks up the cylinders. At the time I had the ammo and not a .38 Spl revolver.
I have a Marlin 1894SS in 44 mag and although guys claim you can shoot 44 Spl, I see no reason when you can shoot low end .44 Mag reloads.
Same for a 45-90. I you had a 45-90 and wanted to do so, shoot reduced (45-70) loads. In all honesty, I don't see the need for expense o converting the gun to 45-90 when that Ruger will take 45-70 loads to place where you will regret shooting it, recoil wise.
