Often times they are just stuck and it takes a sharp rap with a rubber mallet or other tool like maybe a piece of wood
to break it loose, don’t use a steel
hammer as many manifolds are cast and break rather easily.
I would be very careful with the pry it off method, it’s very easy to damage the mating surfaces, so if you do it be very careful, and don’t pry, back to manifolds can be brittle.
Never tried the freezer method myself, but why not give it a go, a lot less aggressive than most anything else.