Well, there's two options:
1. computer or computer operator who looked up the plugs screwed up or:
2. Aftermarket cylinder heads, which if you car is heavily modified there's a chance they may be different.
It's most likely 1 because even in most auto shops and parts departments theres always mistakes being made and information misread, etc. I believe nearly all 5.0L's take plugs ASF-42C (ford number), except for maybe the new explorers with the GT-40P heads, but i don't know the autolite number.
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'84 Mustang 5.0 LX
My car