Well the good news are that the massive fireball means you have both spark and fuel. The bad news are that you are firing the fuel while the exhaust valves are open. So that makes me believe the distributor is put together the correct way from whoever rebuilt it. I would start over and go through the following steps:
1. Put the engine at top dead center and take the spark plugs out.
2. Stick something long in cylinder 1 (the one nearest to the cam gear and further away from the distributor).
3. Turn the engine clockwise with a socket wrench at the crank and make sure that when it comes back around to the marks the piston in cylinder 1 is at its highest (note that for every rotation of the cam, the crank rotates twice). This way you will be sure that the engine is timed correctly.
4. Don't put any sparkplugs in.
5. Put the distributor in with the sensor facing up, make sure that the engine is at tdc for cylinder 1, mark to where the rotor arm is facing, put the distributor cap back on and plug cylinder 1 wire to where it corresponds. No other wires need to be plugged and you need to unplug the blue sensor.
6. Put the spark plug on the wire and set it on the engine somewhere, where it gets grounded and you can see it when it sparks (cam cover may or may not do).
7. Turn the engine clockwise from the crank and keep an eye for when the sparkplug sparks. It should spark just a couple of degrees before the timing marks align (6 degrees if I am correct). If it does that means everything is set right .If it does a couple degrees off rotate the distributor a little. If it doesn't spark anywhere near it, mark where the cam gear is facing when that plugs gets spark and we ll have something to go on with.