I recently removed the head on my 216 to have it checked for cracks (none were found). I put everything back together with new gaskets, gasket sealer, and the proper torque for the head and manifold bolts. About a month later it developed an exhaust manifold leak. When I checked, many of the manifold bolts were pretty loose, so I tightened them back up. This worked for a few days, but now I can hear a leak when I first start it up. After the engine warms up, the leak stops. The manifold bolts all seem tight. I did not have a problem with exhaust leaks before I took the head off (the machine shop milled 6/1000 off the head, if that matters).
I'm wondering if I have to replace the gaskets once they start leaking.

My dad always told me, "Don't force it, get a bigger hammer".