I have a 45-70 -- a great and timeless round, and ample for deer, bear, and elk especially if used within its range limitations.
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Deer 300, Bear 500, Elk 800 ft lbs energy......

My question is not the cartride nor the rifle but the minimum energies as stated.
Though certainly not a flat shooter, a 45-70 govt STILL has 800 or so foot pounds at 400 yards after falling out of the sky many feet.

So, at the gut level, does the energy remaining (800 lbs or so) in a 45-70 at 400 yards (assuming you could connect at that range) provide enough giddy, up, and go to cleanly kill an elk with a hard cast lead bullet ripping a long and wide wound channel? I' m a believer in the " momentum" component of killing power, but I' m not sure that dog will hunt.......

Fortunately, the limited range of the 45-70 is likely to insure that you are hitting them with much, much more kinetic energy than your stated minimums.
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We have this much in common --- I think your rifle and cartridge are fine --- the momentum factor of killing power is underestimated, and hitting them with much more energy than ones personal minimums is a good thing.

In any case....

Never Go Undergunned
EKM