As far as I can tell there is only one side of the thread missing... the middle side. I offered up the urethane (Gorilla Glue, et al) goo as a useful way to help problem joints, not as an all-or-nothing, everywhere-or-nowhere solution.
Reasoning...
1) When these cars were built, there were no non-brittle glues. Horse hoof and hide glues, melted in a pot, were the order of the day, as per the furniture industry, which were very good until they cracked or got wet. Plastics were virtually unknown except for bakelite, and a few pretty poor thermoplastics.
2) Where many of us are reluctant to replace a perfectly sound wood member because it suffers only from slightly splintered or weathered ends or a bit of surface deterioration, urethane glues can make the your member (perhaps even my member) quite serviceable. It will bed the metal or wood gussets nicely, and allow screws reasonably near original size (stainless is obviously the best choice) to be re-installed where they could never hold otherwise, and yet retain their ability to be removed as needed because the urethane, unlike epoxy, does not have quite the same shear strength and is not anywhere near as brittle.
3) Dry joints (no glue): Rubbing the screw threads lightly across a block of oil-based modeling clay (one side only is enough) is the best way I've found to keep the pilot holes tight enough (small enough) to allow optimum holding strength while reducing the insertion torque enough to insure that the heads won't break. The clay will dry out, leaving a somewhat abrasive film around the shank. It's a lot better than any other lubricant because it's temporary.
4) Given the resilience of the urethane stuff, it does allow some flexure, beyond which the joint would probably fail anyway. If all the flexure were limited to the joints (instead of transferring some of the deflection to the wood), the framing would come apart even faster.
5) Compromise: Rub the end of original, slightly cracked or otherwise stripped screw holes in urethane glue, wipe off the surfaces, and let it dry for 24 hours before assembly. You can then redrill smaller pilot holes and assemble dry (using some clay as above).
6) If there's no reason to glue NEW tub framing members together in most spots, don't.
4) As we all know, some joints are just meant to be stiff. When they're over fifty years old, they sometimes need help.
Austin