A bespoke pair of Earle actioned Creedmoors, WOW! I would think this one may be a lot more fun to shoot than the 45-90, tho any 45-70 can still be a handful. I'm not a BP shooter, but have friends that are and shoot some of theirs from time to time. I have a modern 45-70 Marlin that is brutal by any standard that digests smokeless only. It always seemed to me, as the trigger puller, that BP had a more abrupt and immediate recoil, but that's a subjective best left fettered for the night.
Parker-Hale makes hammer forged Alex Henry rifled bbls. in .45 cal. You may wish to contact them & see if they would make a blank or blanks available to you. I would think if your inquiry was sent to the top & included some photograhs they could hardly refuse. Worth a ranging shot anyway.
Curiosity on my part .. how does your 45-90 perform? Did you get to sort it out & use it in competiton this past season, and if so, how did you shoot?