Some super-duper single shot only nuclear loads in 45-70 can reach or slightly exceed 3500-3600 ft/lb, the 458 is around 5500 with some load approaching 6000.
Of course, your use of energy like an ammo marketing piece overstates the real difference, but it's still a big difference.
The .45-70, introduced a quarter century before smokeless powder was common, was never designed for really high pressures. That's one reason the commercial .475 Linebaugh isn't made from cut-down .45-70 brass like the initial experiments.
The .458 WAS designed for high pressures and modern powders.
And the .45-70 wasn't designed for elephants.
Now a lot of the modern hyper loadings of the .45-70 are a bit silly for most applications of the thing, given that the original black powder loadings will put a bullet straight through a buffalo, where it will kill the next one behind it. Large-diameter, heavy, non-expanding bullets don't need the velocity that a smaller, lighter, expanding bullets require for proper expansion and to overcome their lack of momentum.
The problem with the .45-70 is its trajectory. It can be learned, but it isn't easy to learn. .500 S&W, with even less aerodynamic bullets and less velocity, isn't going to make that any easier. And with either round, you don't need any more
energy, from a rifle.