don't go slow
Hello hoobens. I've looked at this .44 twist rate regards my carbines and have a firm opinion on the matter. Don't go with a 1-38" twist.
The early Ruger .44 Carbines were so rifled,and their reputation for acccuracy was so-so. The Win '94 and the Marlin 1894 as well. Note too that all the .44 Mag revovlers of the same era were twisted 1-18" to 1-20" (from memory)....there were no .44 mag revovlers with the slow twist. None, ever. I've read, and my two early Ruger carbines, and my Dad's Win '94 support the story, that a good .44 revolver could outgroup a Ruger/Win carbine on occasion, and we found that to be true. Yes the revolver was harder to shoot well, but off bags, with a pistol scope, a good M29 or Super B could run with the carbines of the day, shot for shot.
Proof of all that, I believe, is that when Ruger reintroduced the .44 carbine as the Deerfield 99/44 (?) and the lever 96/44, they went with the faster 1-20" twist like on their revolvers. That firmed up accuracy with the "standard" 240 gr loads, and allowed the use of the heavier loads 265 and 300 or more, as well. My 1-38" carbines will not shoot those heavy slugs worth a hoot BTW.
Were I to build a custom .44 carbine as you suggest, I would not roll the dice with the slow twist. There's nothing to be gained with a slow twist, and a chance that it may not do the job with certain loads, likely depending on velocity. I'd go 1-20" or so, as near all revolvers as done. That faster twist will certainly stabilize the standard 240 gr, and allow heavier slugs if you want.