I've used brass nuts on exhaust manifold studs and exhaust flange bolts and studs on everything from farm tractors to sports cars for the last 50 years. Nothing comes off more reliably after decades of rust. Even use them on intake manifold and carb studs.
If you are talking about manifold to head Bolts, a good wipe of Never seize on the threads and any kind of bolt will come out years later.
