Depending on the scenerio, really dictates if these guys are "lining" fish or matching conditions. The 6' leader is exteme though. Maybe a 4-5' maximum. I will use longer leaders (but LIGHTER leads too) on my summer fishing. I usually don't go TOO long. The length of leader dictates how you feel the strike. But these guys running multiple OUNCE weights (which isn't needed on small corkies on long leaders) is virtually asking the fish to run into it. But it is a necessity when you're fishing gin clear conditions with spooky fish. Yet, as i said, small profiles, longer leaders, and usually split shot for weight when I'm driftfishing.
Now, onto kings. I've caught too many to imagine on drift gear. I'm talking rags mostly, some with corkies. Some where caught with bait, but most where caught bait/scentless. Most were solid takes, so anyone saying I had "lined" a fish would have a battle on their hands. But I'm using what is needed for situation. Usually run 3' leaders when lower vis, and a little longer for greater vis. When it comes to flies, I've actively pursued kings on a fly. Have caught a few on subsurface/dry flies near tiedwater. Basically a revised wog I've tied. Cool to see a silver/king come to surface and roll by fly. But that's only been in flux salt zones, not complete fresh. As in others, in the flux, i fish GP's, shrimp patterns, etc. But in the fresh, I go wild. I tie up big bushy flies. I tie up a variety, depending on water I'm fishing. I tie up my dredgers (ones I want to be bouncing off bottom with a sinktip), and my sinking flies. The norm on all is bushy. Lots of marabou and schlappen. Can only tell you about kings I've caught on the Chehalis system, Quielleyette system, and the Hoh. Those are the rivers I fish for Kings. Depending on water clarity dictates color combo, and river dictates colors too. I guess best way to sum up my flies are they're similar to George Cook flies. Usually I put some chenille, maybe some misc materials. I usually don't use traditional hairwing flies. But I've had success with what I use. Plus, as juro said, they have a mind of their own. They hit when they feel like hitting. But usually when they hit, they H-I-T. Never had just a mellow take down. Hell, had my old 10wt Fenwick almost ripped from my hands stripping in my shrimp pattern on the Chehalis. I was reeling like crazy trying to get the line back on reel because the fish taking the excess line out of my hands was burning the hell out of my hand.
I'd say tie up what you think would work, then hit the rivers. You never know.