Moderately priced doubles chambered in 45/70 will most likely be built on relatively modest double rifle actions. These will be suitable for small bores up to things like the 9.3x74r. These are often double shotgun actions. Pressures per se are not so much the issue, its the overall force being sent back onto the action frame.

If you look at proper big bore double rifles chambered for the likes of the 470 Nitro Express the water table is longer and the back of the action around the detonation will be considerably deeper, all of which makes for a much stronger action able to withstand the much larger larger recoil forces. Big double dangerous game don’t produce huge pressures, but they do produce lots of recoil.

Lengthening the chambers of a 45/70 so effectively duplicate 470 NE type ballistics would probably result in the action cracking sooner or later. Besides it would very uncomfortable to shoot as would almost certainly not have enough weight to absorb the recoil.

The 45/70 is a very capable cartridge in its own right, and probably best used for what it is designed for. And in a double rifle would make an excellent close deer, elk and boar type rifle.

But if you need a big thumping DG rifle, get a proper double built for the job, or get a BRNO or CZ bolt action in 458 win with full strength ammo, or have it taken out to 458 Lott.