You get spark, when the magnetic field around the coil, collapses...when you turn your key to off, the field collapses, and you can get a spark......
Now the question is, is this the old points system in the distributor, or was it ever upgraded to an HEI system?
If the original points system, check the points.......I have seen the rubbing block come off, so when the distributor rotates, the points never open....points could also be pitted, open(too big a gap).......could also be a bad condenser....
HEI, either open pickup coil or bad ignition module....