I've owned both the .45/70 and .458 Win Mag Number 1s.
I have a Marlin M1895 that I bought and it outshot the Ruger so it moved on.
The .458 Win Mag does have a long throat. Looks like the on ramp to the freeway. I found two bullets that it shot well. #462560 and the RCBS 45-500-FN. Both seated out to touch the rifling. The 462560 seated normally would shoot acceptable groups.
My thoughts were like yours. Nice heavy plinker off the bench with a 300 grainer. I didn't get the groups that I anticipated so the .458 Win Mag is gone as well./beagle