Used that method recurrently and if it did not stop the problem, did a few other mods involving relieving the hole/slot in the manifold casting to provide additional expansion and also taper ground the outward face of the manifold ever so slightly (thicker at the ends and thinner toward the center). The concept of using the bronze is the softer metal allows the cupped washer to slide under heat cycling (put some graphite under the washer).. A few cases I've actually filed the cupped washer to provide a "flat" rather than a cutting edge.
On some motors we have even gone to soft steel studs with a small nick with a hack-saw to give the studs a spot to break above the head surface.
Lots of little movement ultimately breaks steel that is subject to frequent heat cycling.
Some things are impossible to stop (look at aftermarket 4.0 l jeep motors and their aftermarket accordion tube steel manifolds}. Control or allow for movement is key.