The 289/302 FO is 15426378, so that is so far removed from the 351 FO that if you have it running, you must have a 351, and the FO is not your problem. On the plugs that are not firing, have you removed the wire and grounded it to the block while the engine is running? Did you get a blue spark when you did? Just make sure you are getting spark from those suspected wires. If not, then replace the wires. If so, then take out the spark plug and see if you are getting an arc across the tip. If not, then the plug itself is suspicious.