+1 on checking for all of the bolts. Remember to remove the starter!
If this is an Auto, don't forget the torque convertor bolts!! If it is forced, it will make one big mess when it actually does come apart. Not to mention, damage the pump.
Also try spraying some penetrating oil on the two alignment dowels (10 and 2 o’clock). Let soak. The dowels are steel and rust into the block. One of the dowels is visible in the pictures.
While waiting for it to soak, double check that the engine and transmission are supported in a line. If the engine/transmission is allowed to sag, then the weight will bind on the transmission input shaft making separation more difficult (damage possible).
Support the rear of the engine (block of word on the oil pan). Make sure the transmission is supported and level.
IMO, careful use of a pry bar is advised. It is easier than you think to break the aluminum bell housing. Use your hands to wiggle the transmission back/forth/up/down. Do not hang on it. It may take some force to separate but take is slow as it is possible to damage some real expensive parts.
Good luck.