The original plugs in that engine were Bosch Double Platinum # FR6KPP332S, Rock Auto sells them online for $14.77 each, plus shipping.
My preference for that application would either be Autolite Iridium # XP3923 ($4.62 ea at Rock Auto), or Denso Iridium # 5304 (also called IK20, for $8.08 each at Rock Auto). Iridium plugs have a greater tolerance for heat than platinum; they also require less voltage to fire, and they have better fouling resistance.
The plug gap for that engine should be set to .032". Some plugs will not come preset to that gap.
If you really want to get the most performance from that motor, and are willing to wait for plugs to come from a warehouse in Texas; I would recommend buying Brisk racing plugs from Brisk Racing online. They have two different types of racing plugs for your motor; one has a sliver center electrode; which has better heat conductivity than either platinum or iridium, and thus will be most efficient over a wider range of conditions and loads. The other is an iridium plug, which has a tapered center electrode and a focussed reflector design. That plug will tolerate heat much better than other iridium plugs, and will produce more power at full throttle.
The Brisk silver racing plug is # DR14YS, and sells for $10.99 each. The Brisk iridium racing plug is # DOR14IR, and sells for $14.99 each.
Source: www.rockauto.com and www.briskracing.com
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