Interestingly, Hodgdon's 405-grain bullet Trapdoor loads have some pressures just the same as their levergun loads. I would ask them if that isn't an oversight. I've always heard numbers like 21,000-22,000 psi max for original trapdoors, and 18,000 psi would certainly help keep fatigue down.
Pedersoli's statement that their guns, which will be made with stronger modern steels, pass the same proof as any other 45-70s. The CIP rates the 45-70 cartridge at about 32,000 psi and the proof mean value at almost 40,000 psi. Their instrumentation system is a bit different than SAAMI's, but their mean proof pressure is about the same (39,500 psi is the SAAMI mean proof value and 39,885 the conversion from 2750 bar, which is the CIP mean proof value). Nonetheless, if Pedersoli says to keep jacketed bullets down to 300 grains, that is likely because heavier weights, with their greater inertia, combine with the jackets to provide higher start pressures, and that may well put more wear and tear on the design so that I would stay with that weight number for jacketed bullets. Lead, it appears, can be shot with any safe pressure load up to the 500-grain weight without a problem.
Black powder is fun to shoot but is a cleaning chore.