How do I determine what the gap should be set at for the Autolite 103s I just purchased.

You need the gap coin, like $1 at auto store.
From ClubGP:
Edit: I'd go by these guidelines:

Non-intercooled:
3.8” factory: Autolite 605 @ .055-.057” gap (or AP605/APP605 platinums @.060”)
3.4-3.25” pulley: Autolite 104 @ .052-.054” gap
3.1-3.0” pulley: Autolite 103 @ .047-.050” gap
2.9-2.7” pulley: Autolite AR94 (4 range colder race plug) @ .044-.046”

Intercooled
3.25-3.0” pulley: Autolite 104 @ .050-.052”
2.9-2.7” pulley: Autolite 103 @ .046-.048”
2.6-smaller: Autolite AR94 @ .044-.046”

ClupGP gap

I have an 2002 also. I just got AL 104s. I gapped it at .053"

Should I return the 103s and get the 104s, I noticed that for my pulley size they said:
3.4-3.25” pulley: Autolite 104 @ .052-.054” gap
Can I still use the 103s?

Yah you can. I don't know the perfections to it all. I def think someone shall chime in on that soon. I think some are running 103s.