375 HP 408cui qustion

Yep, something seriously wrong in the bottom end. Disassembly and measurements required.
Should turn over fairly easily with a 1/2 ratchet, easily with a breaker bar/torque wrench. (Assiming it doest have 800 lb valve springs, lol)
A good starter/battery combo should tip it over sitting on the ground.

Edit: if the stroker crank has full filet rod radius, and standard non-filet bearings were used, yeah, it'd be real difficult to turn.... and put LOTS of metal in the oil.