On recoil: It is not that I am so much a wimp as that I am getting careful. I have a friend that got a detached retina from over indulgence with a hot loaded, light weight 45-70. Not a gun to take for a fun day at the shooting bench.
This is a good point. All too often I read statements on here to the effect of "man up" or "grow a pair" when a poster asks about recoil or remarks that the recoil of a particular gun or cartridge is unpleasant, as if the ability to withstand heavy recoil were some measure of masculinity.
I shoot because I enjoy it. I hunt because I enjoy it. I assume most folks here do the same, so I'm perplexed by posters who mock others over recoil concerns. My philosophy is simple; if you don't enjoy shooting a gun, you won't be as accurate with it, because you'll dread the recoil. That's a perfect gun to trade for something you will enjoy shooting. You'll shoot more and be a better shot in the bargain.
I traded the HR 45-70 because I didn't enjoy shooting it. Even with milder loads, the light weight and short barrel of the gun gave it more recoil than my 375 H&H Browning A-Bolt Medallion. I traded off a perfectly good Winchester 70 in .300WM for the same reason. I keep the big Browning because the weight of the gun, the port, and hand-loads leave it with little more felt recoil than my .30-06 700 with the 20" barrel and hotter hand-loads.
Everything is relative. I had and traded a Marlin in 45-70 for much the same reason.
To answer the OP, yes the HR is strong enough to take heavy 45-70 modern loads, but you might want to work your way into them once you're shooting the gun. You can download the .45-70 to levels not at all unpleasant, but by the time I could have downloaded that far, I was ahead to go with a different cartridge/rifle altogether.
Just my two cents worth,
KR