1974 Chevy Monte Carlo wierd timing

I purchased this motor. It had been rebuilt by a shop. The guy who i bought from didnt like the cam. So he swapped it out. He had money issues so he sold it to me. So i put it in my 79firebird. Put in a new distribitor. Tried to set timing where its suppoose to be. I position the rotor in the direction of #1 cyl. And ran it but it ran poorly. So i moved one tooth clockwise. So the rotor was pointing at the fan.[ 6:00 posion]. It ran a little better then but not like it should. So after i scratch my head and had a buddy over to help me out. We checked the wires [order] everything several times. So just for fun we went one more tooth clockwise. Towards #2 cyl. Then it ran almost perfect. This blew our minds. Now does that mean the timing marks under the timing cover are off a tooth??? And is running it damaging anything???