The Stovebolt Site Forums The Shop Area Engine Shop 1954 235ci, Stuck Heat Riser and Manifold Question

I would unbolt the intake and exhaust manifolds. It looks like a combination of fuel residue and rust. You could have a machine shop dunk the whole thing in their hot tank, which should remove the glop and possibly free up the heat riser as well. Another possibility for removing the glop would be sandblasting. That would give you a nice surface to paint.
But the heat riser can be freed up by continuing with your heat, tap, PB Blaster method. It'll eventually free up.