While the actual manifolds on our '66 Mustang were in good shape, we can't say the same about the studs on the passenger-side manifold. Only a handful of threads were still usable on each stud, so we needed to remove them before restoring the manifolds. New studs will be installed later.
Merv Rego says there are two ways to remove rusty studs from exhaust manifolds. By heating the studs and the area around them, you can often break the studs loose with Vise-Grip pliers or a pipe wrench. This worked perfectly on one of our studs. The other one snapped off, forcing Merv to switch to Plan B-drilling out the broken-off stud. This requires some careful surgery to prevent damaging the interior threads, but by starting with a small drill bit and working up in size, you can remove most of the stud material before cleaning out the threads with a tap.