Remove the distributor from the head and pull the adapter shaft out as well. Turn the adapter shaft 180 degrees and reinsert it into the head - make sure it seats fully. Turn the distributor shaft 180 degrees as well and reinstall it onto the head. Reset the distributor to top dead center and try to start the engine. If it backfires, then the engine is 180 degrees out of time and you'll need to reverse what I described above. If the engine starts, then you can fine tune the timing - if memory serves it should be 4 degrees before TDC.
It could also be that the plug wires are not properly installed. On the early engines, plug wire #1 sits immediately to the right of the cap hold down clip, with wire #5 to its right. That hold down clip faces the front of the car - at least it does on my 50K.