A bit OT, Home rnovation question

Sorry this is OT , I just figured there are enough experienced people here to off opinions. I have a 75 year old house with a living room that has a cathedral ceiling about 16ft high.. About 70 years ago they built an extension on the side of the house that buts up to what used to be the gable wall. I want to cut a large opening thru this "gable" wall, about 10' X 10'. That way when you enter thru the front door the living room will be more part of the entrance hall. There are no floors attached to this wall on the extension side , just a rafter of the other roof that runs the same as the LR, but a few feet lower. To my mind this gable wall is not carry much load, because there are interior walls supporting the roof rafters about midway before they reach the outside wall plates. So the forces are pushing out. the ridge beam in the living room is about 2" thick and spans the whole length , 25', unsupported, except at the ends, there is quite a sag in it now, not noticeable from the inside, but I have seen it when I was up on the roof. So I'm looking for feedback, of course I will install a header, but how careful would I need to be when cutting? That wall is sheeted with diagonal 1X6 T&G pine, as are all the exterior walls.

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