Does your rear axle have another vent ? I thought that brake line junction was secured with a special bolt that is drilled and serves as the axle vent.
I added some vertical pieces to a similar design so it created a pocket for frame horn then I ran a horizontal bolt thru. Mine has the og front cross riveted in so has more frame twist up front, your front cross will handle that.
Take pan off, I realize you explained no stand so may have to wait but I would look in there and inspect the front drive/timing assembly. PITA to open all that up but worth it. See if a valve dropped into cyl.
I'm in Nor Cal and my 40 sits outside , uncovered for 23 years, no top, only real rust is the leaf springs and a tiny bit where I smashed a rocker, so lucky, I have zero tolerance for that s***, I'd buy a new whatever it is unless it was unobtanium , then I'd hire someone like you to solve my...