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I wouldn't try and turn it. Not with all the valves and lifters like that.
Pull the cam gear and see if the pistons want to move. Loosen the rod bearings so there is free play and see if you can move the stuck pistons within the free play without having to turn everything else.
If you can pull the crank and pistons/rods, you can then tackle the valves. Just look at one at a time, you should be able to get them apart. Then knock the lifters up from below and pull the cam.