Take off the passenger side valve cover and watch the front two valves.Roll the engine over until the exhaust valve opens and closes.Next the intake valve will open and then just as it starts to close watch the timing marks on the balancer.Roll the engine until the valve closes and the timing marks line up. This is #1 cylinder at top dead centre.Make a mark on the outside of the distributor that lines up with the #1 spark plug wire on the cap.Turn the rotor until it lines up with this mark. Now drop the distributor in with the vacuum advance towards the front so it has lots of room to move to either side for later adjustments.As you drop in the distributor you will notice the rotor will turn in one direction as it mates with the cam gear.You will probably have to pull the distributor back out and turn the rotor backwards a little and try again .It may take you a couple of tries to get the rotor to line up with the housing where you want it...If the distributor won't go all the way down just hold downward pressure against it and bump the engine over and it should fall right in.
Did you mark where the rotor was pointed when you removed the dizzy? If you did not, you will have to start from scratch. First, remove the #1 spark plug. Stick your finger in the hole and crank the engine over until the compression blows your finger out of the hole. Look at your timing marks and line up your pointer with TDC. Look on your cap where the #1 plug wire is located and mark this spot on your dizzy. This is the spot where you want the rotor pointing when the dizzy is installed. The rotor will turn alittle as you install the dizzy. You may have to turn the rotor a little one way or another to get it in the right place. Now comes the fun part, The oil pump shaft incerts into the bottom of the dizzy and sometimes it is out of alinement. If you can't get the dizzy to drop down all the way lets you know the oil pump shaft is not in place. You may have to turn the rotor one way or another to get it to drop down. Remember your dizzy housing will turn after it is installed, so you may be able to line up your rotor mark with the rotor without removing and reinstalling the dizzy.
Bump the starter untill you begin to feel a WOOSH... Immediately stop. (you will need someone else to bump it unless you have a remote starter jumper)
When you feel the woosh, immediately look down into the timing marks and with a wrench back the motor up to TDC then 1/4 turn before TDC then back to TDC. (that takes about 10 cesonds)
Place the dist in the motor and just point the Rotor any which way until it snaps down so you CAN SEE HOW FAR THE ROTOR TIP MOVES WHEN SLIDING ALL THE WAY DOWN.
PULL THE DIST BACK UP> MOVE THE ROTOR POINT FOREWARD the SAME DISTANCE before the #1 in the dist cap would be, that the rotor twists when fully seated... Remember the amount of travel on the rotor tip from touching to fully seated, that's what I'm talking about.
WHen you have the point at the approximate place, slide the dist gear into the cam gear untill it stops. If it stops short you should be about 1" to the LEFT of the #1 position on the Cap. (while this is going on hold the vacuum buld in the center of the adjustment radius allowed for movement)
If it didn't stop short and slid all the way down that's GREAT! (but only as long as it points to #1 on the cap.
IF It DID stop short and is still the 1" give or take from the #1 pointer on the cap's position that is good, all you must do then is bump the starter a time or two and it will drop down on the oil pump shaft without getting out of the adjustment you wanted.
Now you can re-assemble and start and set the timing.
I hope that was clear... It only takes about 5 minutes...