Flywheel won’t spin

I ripped out the engines of two ford fusions to
replace the one because it had blown,
everything went fine until I went to connect
the motor to the others transmission. As
soon as you put the flywheel on and go to
connect the two, is as if the crank shaft is
seized. So we ripped apart the engine and
transmission again, both move as free as
butter. As soon as we connect it again, it
seized up. You cannot move the crank shaft
at all. Anyone know what’s going on? We
checked multiple times, nothing is preventing
it from moving as far as we can see...