I'm trying to replace my rear subframe bushings.

I'm having trouble with the main bolt. The small ones came out easy. The main bolt turns about 20 degrees, but spins the holding plate with it. If the plate is locked in place, the bolt won't turn. It is as if the plate and the bolt have rusted or otherwise locked together.

What is my friend here? PB Blaster? Heat? Great honking breaker bar? What damage am I getting ready to do?

Thanks,
Bob
'82 300D