Keith,
A big 6.5' monster Eastern or Florida is one of the coolest snakes in NA IMO - pretty, tough as hell, hardy, easy to care for, and just great all around. That's why I've got 12 right now!
That was a cool account; I have no doubt that ounce for ounce or pound for pound a king is the toughest snake in North America and will kill a YT or Indigo 80% of the time if the are near the same size.
But a 8-9' Cribo or Indigo will likely kill a 6.5' or 7' King unless the King managed to avoid being bitten on the head and got a wrap - then I think it would be a brutal battle with one killing the other or both being mortally wounded, or both going their own way.
I've held and seen 7.5 foot Indigos that are so large around that it's amazing and holding one of these 9-10lbs snakes is wild. A Dry. of that size will kill even a large Eastern most of the time IMO because they are just so heavy-bodied and have such girth and huge heads and such power in their jaws.
So a Dry. that size I think are the baddest snakes in NA (in South America Mussurana will kill even the largest YT or BTs).
Now if you find (or grow) a 7.5 foot, 9lbs Eastern or Florida then it would be a really tough fight.
Now, having said that there is no doubt that kings are more successful snakes as a group in North America than Indigos are, or have been, for 10s of thousands of years and biologically speaking are one of the dominant group of snakes in North America. You just have to look at the range and how many different spp. or Lampropeltis to see how dominant they are. And when you consider that they are snake eaters, the baddest group of snakes in NA!! I just think some Cribos or Indigos get too large for even the largest Kings...maybe I'm totally wrong and kings could even kill a YT or Dry that's longer and heavier than they are. I'm not taking a chance with my snakes though!
-John