How high do you currently rank King? Peak High Tier? Inbetweener? Low-top tier?
I don't place have a firm placement for King on a tier list, but I currently have him at least one class above Marco and Katakuri.
I think he can beat either of them with some level of medium difficulty (in the sense that he would have negligible HP damage and minimal stamina lost when he wins the fight).
If you say Marco and Katakuri are both High High Tier, then I would say King is a Peak High Tier or "In Between"/"Bottom Top Tier".
I would probably elevate King to Low Top Tier if he shows Awakening or Haoshoku Coating.
Alternatively, if he can contend head on against Zoro's own Haoshoku coating.