Modern stocks fit different than ML stocks. It will not work out well. For off hand shooting with a long barrel you want a shorter stock than you would with a modern shotgun. I made this mistake for myself.
I am 5'10". I made my first complicated LR with a stock length as a modern shot gun would be, 14 3/8". IT turned out much longer than fits me. During the build it seemed good. Later I found that I hate it, I never shoot it. The trigger reach is much to long. I can not shorten it due to the patch box, carving and cheekpiece. IT feels very muzzle heavy.
For me, a pull of 13 1/2" is about perfect for me on a LR. For best comfort and least strain I want my right arm between the shoulder and elbow to be 90* from the centerline of the stock as viewed from above. If I have to reach further than that I can not do so comfortably. Using a good strong off hand position the rifle is laid across the chest. It is not pointed out and away from the chest. I can not make my arms from shoulder to elbow go past 90* from parallel to my chest. That limits stock length.
IF you are shooting a long stock and it feels OK check you position. You probably have a shotgun type position where the gun is facing away from the chest. Instead you want it way over to the left as is a good off hand target position.
For off hand shooting shorter is better. First you will not be straining to reach the trigger. Second, it moves the rifle rearward, that puts your left hand closer the center of gravity. IT will be less muzzle heavy. You will be able to hold it steadier for a longer period of time. Yet you get the benefit of a long barrel which has a lot of momentum to resist the wobbles.
Hope that makes sense, basically shotguns are used differently than a ML rifle, the stock dimensions do not translate.
