We took our 96 LTD in to our mechanic for a service as she was running a bit rough intermittently. He rang me shortly after to advise that one of the spark plug leads was so close to the extractors that it was arking out. He asked if I wanted new leads and plugs, so I agreed. He had trouble getting the right leads as they had right angle connections on them, but eventually got them from Ford. The new connectors were too close to the extractors so he repaired the old lead and insulated it from the extractors. When it came to the plugs, he ordered the standard replacement plug for the 302, but they were way too short, and not wide enough to fit. After cross referencing the plugs that were in the head originally. he got a replacement. He didnt know much about V8's and didnt know why the plugs were different. I gather the heads have been changed at some stage, but how can I tell what they are?? We have owned the car for probably a year and half, and have poured a bit of money into the old girl as it was neglected by the previous owner. Someone has spent money on it in the past, with the engine work, lowered all round (too low in my book, but still legal), and a great DVD system. Ring bells with anyone in Sydney??? Anyway, my main question here was what would the heads be, and how can I tell what they are. They take a larger and longer plug.
Any ideas appreciated. Thanks

Take some pictures the engine setup for us, ie cylinder head's, spark plug setup, photo's witht he engine covers off, etc etc, it'll help a lot!

If you mean the thread diameter of the plugs was too small to fit, yet they were correct for a 5.0L? Then it's most likely got early ('60's-'70's)windsors head on it. They had the (annoying)large thread spark plugs.

Yep, old ones were 18mm, and the later ones were 14mm.

I have tried to take a photo but too much gear in the way. All I can really see is the rocker covers, and they are unbelievably clean and no marks on them. The old girl has been looked after well.
The plugs that were fitted to the heads were wider and longer. For what reason would someone put old heads on it, if that is what they are??