Well- Mopar Rust Penetrant (sp?)- available over the Parts Counter at any Chryler/Dodge/Jeep/Fiat dealer. Used to be called Heat Riser Valve Lubricant. It has a blackish spray. Basically PB Blaster with Graphite in it.
Soak it down- wait OVERNIGHT- and it should came loose.
Never had it fail yet- even on bumper bolts that were 40 yrs old and in a junkyard for 35+ years....

Good Luck,
CC.