I have had an ongoing difficulty with a slightly warped manifold. Flat gaskets or ring/glands - still leaky. New exhaust manifold from Snyder's fit perfectly and even the intake seems better seated. Best money I've spent on the 99 year old T! Even iron wears out sometime.
Check this thread out on Solid Copper Manifold Glands
I should have added that the old manifold ports were rough and uneven. I have no way of doing a fix in this case. DJH
My exhaust ports were badly eroded from exhaust leaks over the years. My rebuilder had his machine shop source weld up the ports and re-machine them. Don't know the material used.