Polarity on the pick-up wiring was my issue when this happened on a 360 with a MSD (I know you said you already checked this). What about the wiring itself? Unplug it at both ends and check both leads for a short between them and check both against body ground as well.

Next thing I would try is a known good ignition/coil/distributor wiring it up completely with new wire isolated from the existing.