That photo looks VERY familiar.
I recently removed the parking brake assembly myself, but the clevis pin you're having trouble with wasn't rusted in place. Managed to tap it out. The cotter pins on the end of the pedal shaft however were a different story. Wound up drilling them out once I had the assembly out of the truck.
The suggestion of trying more heat is a good one. You can get the sucker red hot if needed.
You could also just cut the clevis to get the assembly out and work on the pin later. Clevises, pins and rods are replaceable, and they may be unusable anyway. The rod is just round stock with threaded ends and easy to make with round stock from the hardware store. I wouldn't try welding it back together if you cut it. Clevises are available from McMaster Carr and other places. Sometimes it's better to just sacrifice a few parts to the rust gods.
