Describe what you heard when it was running ... and what it sounded like when the engine seized.
Chances are if a motor seized while running, there would have been a catastrophic sound just before. The likeliness of it seizing after running it then keying off and never starting again seems a bit too suspect to me. IF anything, I would suspect the starter turned over, started the motor, but the bendix gear failed to retract back out of the nose-cone and "ran" with the motor for that short time - heated up like all hell, then the nose-cone bushing seized up from all the heat effectively locking the bendix gear in place.
Try this if it's not too much trouble - remove the starter, visually check if you see the starter motor bendix gear in the "kicked out" position, and try to spin it by hand - if it doesn't move, I think you found your problem). If you hear a snapping 'click' when you took the starter out and the gear looks like it's in the right place, then try turning over the engine by hand again with the starter off. Nose-cone bushing failure in starters is incredibly common and a bendix gear stuck "out" and engaging the flex plate teeth would "lock" and engine because you would literally be fighting the starter motor that's locked in place.
I hope it's something as simple as that for ya - otherwise it'll be a case of removing all the
plugs, dropping some amsoil or Kroil down all 8 cylinders and hoping it breaks loose - but I suspect my theory is a bit more sensible. Something like gaskets and
plugs/
coils shouldn't do anything close to what you're describing unless the
plugs you installed were grossly long and smacking pistons (which you would have heard within half a second of cranking before it fired up).
