crow feather,
Welcome to the world of black powder cartridge rifles (BPCR) and the one single shooting hobby that may well consume you and your other shooting passions.
Best single first choice for entry level BPC Rifles is .45-70. Reason being affordability of components, reloading dies and resale value.
Rifle action type and manufacturer are personal choices tempered with lots of discussion with other users, whether it be first hand or via the internet.
The .40-65 is a good second choice because of less recoil, inherent accuracy, affordability of components, reloading dies and being easily made from .45-70 brass.
Being able to handle .375 H&H recoil is a good correlation if one understands that the average BPCR silhouette match consumes about 60-70 rounds, including sighters, shoot-offs, etc. The correlation ends there, as most persons do not shoot .375 H&H rifles anywhere the number of rounds consumed in the average BPCR Silhouette match. Having shot many .375 H&H, .458 Win Mag, .45-70 and .40-65 rounds my experience is your shoulder and jaw will be much more confortable with a .45-70 or a .40-65.
Others may have differing opinions.
From the Land of Enchantment,
ShortStake