Best Penetrant to Remove Differential U-Bolts?

We needed the leaf springs attached to raise and lower the vehicle while fabricating the "turn the vehicle 90 degrees on its side" type "rotisserie." With blocks of wood already under the frame in front of the leaf spring bolts (front), there is no room for a floor jack anywhere else. Foto is the sample the carpenter is using to follow this weekend to fabricate something similar for me.

wooden rotissiere 12.png