As everyone said don't put a lot of force in it.
-Try pouring diesel and the ATF oil on the cylinders through the plug holes, you can leave the atf overnight it has good detergents.
-Try to rock it back and forth with a big wrench without intake and without sparkplugs of course and don't do too much force it should slowly start turning unless it's reaaaaly stuck with corrosion inside or bird seed maybe?
-Watch out for bearings but for a bearing to be so stuck that could probably have damaged the crank.
-Valve-Piston hitting be careful with that too on one of my older E21 a 316 euro a spring broke and the valve dropped inside the cyl at idle. Be careful this is not the case it as strange as a chain been broken but you never know.