Had a 261 stuck. Kroil for a week, 1/2 diesel/transmission fluid for a week, marvel mystery oil for a week. STILL STUCK.
Pulled heads and found the cylinders slick and clean. Pulled the side cover and found 3 lifters stuck....mainly from condensation. Soaked with Kroil 3 days and they freed up.... and the motor turned.
Hope you have success with yours. It does take time and patience.