Howdy
SAAMI spec for maximum pressure for the 45-70 is 28,000 psi.
My old Speer reloading manual, 13th edition lists three different sections for loading the 45-70 with Smokeless powder.
The first section is subtitled Trapdoors. These loads are specifically for original Springfield Trapdoor rifles, modern replicas, and original Sharps Model 1874 and Rolling Block actions. All the loads in this section are kept to 21,000 CUP. This section also states that modern factory 45-70 ammunition is loaded well below the Max pressure of 28,000 CUP (yeah, that's what it says, CUP, not PSI) to be safe to shoot in the old, original Trapdoors. Dunno whether I believe that or not.
The next section is labeled Lever Action. These loads are specifically for newly manufactured Marlin Model 1895 rifles, Browning Model 1886 rifles, replica (not original) Sharps Model 1874, original Winchester Model 1886 rifles, and original Winchester Model 1885 single shot rifles known to be in good condition. The pressure limit for these loads is under the 28,000 SAAMI spec of 28,000 CUP. Yeah, CUP again instead of PSI.
The last section of 45-70 loads is only for strong, modern rifles, such as the Ruger No. 1 and No. 3 single shots. This section also lists modern manufacture Browning 1885 single shots and Mauser M98 'properly converted to fire 45-70 ammunition'. The loads in this section approach 35,000 CUP. Clearly not for the old guns.
I checked both the Uberti and the Pedersoli web pages regarding what ammunition would be safe to fire in their versions of the Trapdoor. As I suspected, there was no information regarding this. However ALL firearms manufactured in Italy are proofed in government run proof houses, and European proof standards are actually slightly higher than American proof standards. The question gets asked all the time whether Uberti revolvers are safe with SAAMI spec loads or whether they must be restricted to 'cowboy' loads. The answer to that question is that they are all completely safe with SAAMI spec ammunition. I suspect the same is true of all the rifles imported from Italy.
However bear in mind that the Trapdoor carbine is a light rifle. The Army reduced the original charge of 70 grains of Black Powder to about 55 grains because the recoil from 70 grains was just too much in a Carbine. I can attest to that, having fired full 70 grain loads in a friend's replica Trapdoor carbine. If I was going to be buying ammunition for a modern replica Trapdoor carbine I would be careful how much recoil the ammo was going to generate. I would go with ammo with lighter published energy.
Personally, I only load 45-70 with Black Powder, for my original Trapdoor rifle and my Pedersoli Sharps. They are heavy rifles and 70 grains of Black Powder under a 405 grain bullet recoils pretty good. I don't like a 500 grain bullet over 70 grains of FFg.