Assuming the replacement manifold isn't damaged or flats rusted/uneven, engine mounts are OK;
If you're re-using the studs and nuts that might be the issue. Corrosion will reduce the size over time just enough to make them "loose"/lose thread engagement %.
Try and replace the studs and nuts with new, if not changed. Be careful always a chance to snap off.
If long enough, might put a split lock washer or spider type under the nuts.
Make sure to torque them, not just tighten.
There's also those lock nuts that have the dimple, not the insert, that may help. But may make it tough to remove later.
The best loctite is rated about 350F, which is borderline but not far off. I don't think a dab would hurt, even if it bakes off over time.
Also other high temps;
https://www.vibra-tite.com/threadlockers/high-strength-threadlockers/hot-lock-extreme-high-temperature-threadlocker/This Post was from: https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?post_id=257270