Stephane has too much confidence in me. I seem to recall a change in the plugs during the fox run. I can tell you at face value that the plugs on my 88 and 94 are different and I *think* my fox has a fatter plug (it definitely has a huge head - 13/16" IIRC, and no washer surface attached to the head. It's just a bolt with a plastic washer).
If your fox plug is from a 91+, there's a good chance it will work. If it's from a pre-91, I would not be real confident about it.