Severe timing /cam issue

Hello all,
When I accelerated my '63 Valiant to pass another car, the /6 225 engine suddenly began a horrible clanking/ banging noise as it kept running but with only a little power. I pulled off the road, but just as I was about to turn off the ignition (the engine was idling very loud and rough),it felt like a piston hit a valve (or some other mechanical obstruction).
It seemed to me maybe a timing gear issue. I pulled the timing cover off and everything down there looks ok. No damaged teeth, the key on the crank is intact. The chain didn't seem loose enough to possibly jump. The alignment post on the cam was intact.
I pulled off the head, and nothing strange there either.
But, with the head and the timing chain off, I notice that I cannot turn the cam pulley using a 1/2" drive on the cam nut.
Shouldn't the cam turn without great force?
Before I pull the entire engine, was hoping to understand what might be going on.
Thank you for any input.
Confused