Check the magnetic pick up in your current distributor with a decent Volt Ohm Meter and then check your BB ones also, it should read between 300 to 500 Ohms on between the two wires going into the distributor pick up is what I'm remembering now.
A quicker easier way may be to take one of your BB distributors and hook it up to your distributor wiring going to the MSD box and spin it by hand with the coil wire removed from the distributor cap so it can jump to ground and see if it will fire the box and coil
If so it is the 318 distributor, if not then it is the box or coil