Rotate engine until TDC mark on crankshaft damper aligns with timing pointer.
With spark plug #1 removed, stick your finger in the spark plug hole and see if piston is at the top of the stroke and both valves are closed. If not, rotate again until above is achieved.
Insert distributor into block with cap removed so you can see where the rotor is going as you slide it downward. The goal is to get rotor to point directly at #1 tower on the cap WHEN THE DIZZY IS ALL THE WAY DOWN ON THE BLOCK. (it may be helpful to put a chalkmark on the dizzy housing showing where #1 tower is).
If the dizzy won't go all the way down, you may be getting hunk up on the oil-pump shaft.
I DO NOT recommend cranking the engine to align, because sometimes the shaft is just rotated a little too far, so that you end up getting the dizzy in one geartooth off TDC.
I recommend getting a thin-wall hex-socket attached to a long extension bar to turn the oil-pump shaft, then try to put the dizzy in again and try to get it all the way down.