Start by sawing it lengthwise as far down as you can go.
Bust off half with a chisel.
This one's almost off. The other side is next.
Take the blade out of the saw frame, and cut parallel trenches in the fitting. Pay close attention so you don't cut into the manifold threads - you want this to seal again!
Make a few cuts around the inside of the fitting. Then start banging around the outside of the fitting with the chisel. When you find a weak spot, collapse it inward. Work your way around it, peeling it off the inside of the hole, and eventually it will fall out.
Look at those nice intact threads! Clean with a wire brush, ,and chase it with an NPT tap if you have one.
All ready for a new fitting. Use brass so the next poor bastard doesn't have to go through this too.