probably a fair amount of corrosion between iron cylinder and aluminum head.
Presumably you already have manifolds, fuel injector pipes removed plus external oil lines and cooling baffles removed.
Patience, heat, solvent, rubber mallet, start by removing the studs completely and lay the engine on its side, soak well with penetrating solvent (inside and out if possible through your ports), apply heat externally to the heads to cause expansion, whack them good with soft hammer plus wooden blocks. Usually what happens is the cylinders will pull off the pistons along with the heads. Then you can work them from inside and outside the bores.