Release the four locking tabs on the distributor with a flat tip screwdriver.  Remove the distributor cap.

Photo to help you with the locking tabs.

Now you can verify that your engine is at Top Dead Center.  The rotor button will be facing ignition wire #1.  Mark the base with a piece of tape.  If the rotor is pointing to the passenger seat, then the engine is 180 degrees out.  You will have to turn engine one more turn to achieve TDC.

You can mark the base of the distributor and the block for reference.

Reference mark being pointed out.  You can also mark the rotor mark on the base of the distributor.

Remove distributor hold down clamp.

Remove the distributor and pay attention to the rotor as it will spin.  Mark it when it stops with another reference point for re-assembly.

All the markings on the distributor are for reference only since we are installing a new one. The markings will get you in the ball-park for the installation of the new distributor.  Transfer the marks to new distributor.

All the parts we are dealing with in this replacement.

Reassembly

Mark new distributor roughly where the marks were on the old distributor.  This will get you in the “ball park” for the new installation.

Install the new unit, line up the rotor on the second mark and drop it in. Do not forget the round gasket; it goes at the base of the distributor.   Pay attention to the rotor - it should rotate back to the #1 marked at the time of removal.  If it won’t drop in, then rotate the rotor a small amount and try again. There is a slot that fits into the oil pump drive and they must line up. That’s why we made all the marks at the beginning. 

Once in place, all the marks should be very close. If not pull out the distributor and rotate rotor back or forward slightly and re-insert fully.  Re-align all the marks and slide it in, you may need to “wiggle the rotor” to align the teeth on the gears, once in it should line up to your mark, or close to it , if not just pull it out and turn the rotor.

After you have the distributor re-installed the timing will have to be checked again. 

Top left pic is most of the Vettes in the C-3 range.  Notice the #1 on the distributor is facing straight to the front. This is not quite right on most cars - it will be more where #2 is shown.

In this photograph - you can see the white mark I placed to indicate # 1.

Tools used in the removal and replacement of the distributor.