Well there's more than one way to skin a cat. I don't think there's anything wrong with doing it that way. Anything in contact with concrete, or stone, at minimum needs a capillary break (polyethylene or a roll of sill seal.. you could staple it to the wood before you put it up) and/or be pressure treated. If you don't want to grout it then you're gonna need a couple cans of spray foam to really seal up the space between the framing and the brick. Also in 1940 you can bet those guys were using portland. I am guessing you are in the north east based on how eaten up the mortar is on the brickwork. And finally mortar, even today, has lime in it. Portland is the component that they added about 100 years ago. Some guys say you can't use portland when repointing really old stone walls, I'm not sure I agree with that in the north east, though. If it was me, for cost, I'd just go ahead and parge it. If that opening is already very plumb, you might not even need to do the strings like I was suggesting, and just eyeball it. Then you can frame it out. Heck, you could even plumb up a couple 2x on the wall and use that as a guide for parging.