Sounds like a nice project you are starting Yahoody. The 24" HO/HR .50 CAL. with the rifle butstock balances best to me . The 22" RB even with the lighter carbine butplate and stock , AND .45 CAL. bore has a balance point at the brass lifter area . A bit farther back than the better for me junction of forend and frame of the 24" HO/HR .50 . I like the RB , but would choose the HO/HR on your build as you have an octagon cut forend on hand. As for the cal. change , If I shot smokless for the most part I.d say you would be best going with the 45/60 case . Plus it has more metal around chamber area than the 45/75. If BP. was the main powder I'd go with the 45/75 wcf it's great cartridge . I did make the button mag. plug , tube and follower from dimensions taken from Pirkles great book. If you would like one , I'd be glad to make and send you one , my contribution towards your project . Like ! the custom sight set-up idea . Been thinking on a flip-up peep sight on the dust cover . Still scratching my head on it . ,,,DT