There are some engine families where the exhaust manifold bolt holes in the cylinder head are open-ended, you can spray penetrating oil on the back side, directly onto the threaded part of the bolt. You stand a chance of having the oil actually wick into the threads and do some good.
On a SBC, none of the holes are open on the back side. The penetrating oil would have to get between the underside of the bolt head and the iron manifold. This is practically impossible. Then the oil has to travel down the un-threaded shank of the bolt to even reach the threads.
Penetrating oil will make a lovely but useless puddle on the ground.
OTOH, it's potentially useful on the manifold outlets, where the exhaust pipes attach using studs and nuts. At least those threads are exposed.
If this were me, I'd be sure to use a flex-socket on the manifold-to-cylinder head bolts so that no side-force was applied to the head/shank of the bolt. Torque, but no side-force. Either they come out, or they twist off. If they twist off, you can remove the manifold and then soak the studs with your favorite penetrant. However, with the manifold out of the way, I would use a torch to heat the bolts until they glow orange, let them cool, and then unscrew 'em.
Liquid nitrogen? I woudn't know where to buy that around here. I did buy a can of "Freeze-Off"; penetrating oil and A/C refrigerant in a single can. The refrigerant is both the propellant for the aerosol oil, and also supposedly "freezes" the parts to break the rust-bond. Didn't do anything useful on the project I used it for. Very disappointed.
www.amazon.com/CRC-05002-Freeze-Off-Super-Penetrant/dp/B000TFTH00/ref=sr_1_3?crid=YW1FOBJJ93KU
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