Is the firing order 153624 ?
So, you unplug leads 2 and 3 at the cap, swap them over, which sparks now?
If 3 now sparks and 2 doesn't, it is a dist cap/rotor arm issue.
If the spark remains in plug 2, it is a lead/plug cap problem.
...If you want to test the HT (high tension = high voltage) supply to the cap, unplug that central lead (wire?) and put a plug in it. with cranking the engine, do you get regular sparking, or missing out 2 together, out of every 6? If regular sparking, it confirms distributor cap, rotor arm, HT lead, plug cap or plug fault.
If indeed you are missing the feed for 3 and 6, it will be a very odd and interesting fault.