As I was thinking about a solution
I realized I should have listed a fourth thing that may have caused that
bolt to become frozen in your intake.
Gasket Sealer
I rarely use the stuff but it can easily get on the bolt threads and then
be transmitted to the intake threads.
Sooooo ... it may be just one more thing holding that bolt in place
The first thing I'd do is use Liquid Wrench, PB Blaster, or something
similar to see if you can get it to follow the threads and go down into
the intake.
Soak in down several times
Tap around on the bolt a bit to shake things up
Might even heat up the bolt a time or two
Also ... Soak it down from the back side of the hole ... if possible
Anyway ... try and see if all the above will help free up things a bit
Like JT said ... chisel or punch might allow you to get it turning
But if it don't work ..........
Be very careful and put yourself a hole in the Exact center of the bolt
and try to extract it with an EZ Out
DO NOT ... break off the EZ Out
If worse comes to worse ...........
You'll just have to go larger in tiny steps with the hole until you can peal
out the remains of the blot ... thus ... the importance of drilling the first
hole in the exact center of the bolt.
Good Luck Mr. G
Grady