If you can get it to fire up and idle, forget the timing marks for the moment and just start feeding timing to it and see if it starts to run better. If it does, it is very possible the damper ring slipped on the balancer and the timing is way off.

If the timing doesnt help and you can rule out a massive vacuum leak, it sounds like it has jumped a couple of teeth on the timing chain. They will run, but not well. As others have said, pull the valve covers and check to see if you have punched any pushrods through rockers and see that they are all moving and moving about the same amount.