Take my advice...
If you don't know what you are doing when making a clambake (and its obvious you don't) then don't do it at all. You WILL ruin the meal. - So, either go with a clamboil, or better yet, steamed clams if you can find a great, big steamer pot you can toss everything into.
Do not use seawater or seaweed. Doing so is just dumb. The clams as well as other various meats are salty enough as is. If you must, add some sea salt ( from a supermarket, not the ocean ) to the boiling water.
Excuse my total ignorance on the subject, but clams aren't anything that ever inspired my interest.
As old as I am, not even for ten seconds in this whole entire life, have I paid attention to clams.
Anyway.
I was always under the impression clams were an east coast phenomenon, that maybe clams don't even exist on the west coast. An Atlantic phenomenon, not a Pacific one. I've never in my life heard of anyone from California or Oregon or Washington doing clams; they've always been people from New England down to North Carolina or something.
So my impression is we've got all these Oregonians, descendants of Judy grasswire, wanting to be hip, cool, trendy, with it, "eastern," getting the clams at the grocery store.