Seriously stuck heat riser

I have an exhaust manifold with a seriously stuck heat riser. I have tried both PB Blaster and heat with no success. Any suggestions? If I can not repair is there a rebuild kit? Or a source for a replacement?

[Dusty-NZ]

O.K. The manifold is off the engine ,, right ?
This is how I have loosened up a couple .
Acetelyene torch ,, heat the manifold , Gentle tapping with hammer on the heat riser , cold water splash ,, more heat , more gentle tapping , more cold water ,,,,,, more heat ,, tapping a bit harder ,, more cold water ,,,,,,,,,, more heat , squirt penetrating oil ,, cough , cough , where's that darn respirator , suck , suck ,, phew , thats better . more heat ,, tap , tap , tap , more cold water ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, more heat ,,tap , a , tap , tap , WHOOOO , it moved !! , more cold ,, more hot ,, ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
LATER ,,,,,,,,,,,,,,,,,,,,,,, .

[dabenz]

The only thing I can add to Dusty-NZ's good advice is "spit" after "cough, cough." Propane torch also works well if you don't have the acetylene set up.

[kerry]

If you don't have acetylene, put your propane torch on a canister of Mapp gas and you'll get a lot more heat.