I wouldn't waste my money on one of the knock-off distributors, they aren't known for being very reliable.
 
When buying a HEI dizzy try to get one that comes complete with cap, leads and coil. If you can look at it before purchasing check to see that there is not to much play in the shaft either up and down or side to side. 
 
To get the new distributor timed right, start with you old one and pull the cap off but don't loosen the retaining bolt, rotate the engine by hand until the notch in the middle of the rotor button lines up with the notch in the outside of the distributor housing.
 
 
 
On the points dizzy the notch is fainter but still near the vac advance unit. If your old dizzy was installed correctly this mark should also line up with your number 1 spark plug lead, It would pay to check this now as you will need to match this on the new distributor.
 
Now you can remove the old distributor and chuck it in the bin but keep the bracket and bolt that hold it down.
 
Now install the new distributor. When fitting make sure the vac advance unit and the ignition module are clear of the block so there is room to adjust the timing. Once you get the two notches lined up tighten the mounting bolt and on with the cap and leads.
 
Mount the new coil, the green wire from the dizzy goes to the -ve terminal on the coil and the brown goes to the +ve terminal along with your power feed from the ignition. These HEI distributors need a full 12v so if you have either a resistaance feed wire or an inline resistor now is the time to bypass these.
 
All that is left now is to turn the key and fire it up. I would still check the timing to make sure it is spot on.
 
If it doesn't start, first off check you have spark by taking one of the spark plugs out and with the lead connected with the threaded section grounded crank it over, there should be a nice fat spark. If you have no spark check for 12 volts at the coil, if you have power then there is a problem either with the coil or the ignition module in the distributor. The coil can be checked with a multimeter but i'm not sure about testing the module but a google search should turn something up.
 
If you have spark check the firing order and timing and make sure there is still fuel in the tank 

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