Put an electric impact wrench on it (with a high carbon allen wrench head) after donning a pair of safety glasses. Run the electric impact wrench untill you have sheared the bolt.

Now buy new hardware and use marine grease or antisieze on the parts when you re-assemble.

Buy new hardware even if you don't shear the bolt. You may fatigue the bolt with all the flexing it will undergo while forcing it out of there.

GARY