There can be a problem with an early engine -239 /256 maybe? with the number of teeth on the distributor shaft. Hopefully that isn't the case here.
Turn the engine over until the front passenger side cylinder (#1) is at TDC on you timing marker. It is usually well marked on the damper so that you can find it. Due to the four cycle firing pattern - you must be sure it is on the compression TDC - not the exhaust. Many can find this with a thumb over the sparkplug hole - but some need to pull a valve cover and look at the rockers. At TDC on the firing stroke they will both be "loose' - have a small amount of lash clearance.
Firing order is as above and it turns counter-clockwise - so you insert the distributor with the rotor tip pointing at #1 . The orientation shown works well if #1 is toward the radiator - and the vacuum port for the advance can is located at the #4/#5 position. Sometimes they won't just drop clear in - the hex drive on the oil pump shaft below isn't sync'd to the gear teeth so that all aligns. Turning the crank a slight amount - will turn the cam a slight amount - and often it will immediately drop down in place.
When installing a Pertronix - follow their directions - and my advice is to add a grounding wire from the pivoting point plate clear out to the engine block. That insures that as the plate swivels - you can't lose ground to the module and voltage won't be lost to a bad ground.
Steve Metzger Tucson, Arizona