You should not be able to turn the driveshaft in reverse with engine not running. Reverse is actually like a park in that there is a big pawl that locks into drive gear when there is no fluid pressure. Take trucker's advice and pull the sparkplugs and turn it with the fan. Remember there are two crank rotations per cam rotation so that if you are looking for TDC with number 1 firing, you will need to know that the valves are closed for #1 when at TDC timing mark.