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I went through the same deal, soaked it (drivers side) in PB Blaster, WD 40, no help - wound up shearing off the head. At that point I said screw it and installed C5 Z06 springs.
Are you taking the load off the spring? I would recommend doing that if you haven't already. Take place enough pressure on the spring to separate the base pad from the control arm. PB blaster, a careful application of some heat possibly. After that, I might be yanking the spring off and double nutting it or using a quality stud puller to get some real leverage on it.
The spring is off the car.
I soaked it in PB for 24hrs and nailed it with an impact, nothing. I used a breaker bar on the other adjuster and rounded the head.
Anyone have a base model front spring (w/o seized adjusters) that they want to get rid of?