If you have a square bore intake for a 383/400 and a thermoquad carb, just go down to your local auto parts store and pick up a spreadbore/squarebore adapter plate for 20 bucks. You lose some flow with those adapters and a thermoquad, but I really don't think a stock 400 will know the difference. If you don't like the adapter idea, pick up either a stock 400 intake, if you have a local junkyard in your area you might try there, they couldn't want much for it. Otherwise I would go for an edelbrock performer intake. It's pretty much an aluminum copy of the stock intake from what I've heard on here, and it will bolt up your thermoquad.
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Unless it's a 59-60 Chrysler RB383
And that's worth bringing up because there's just soooo many of those around. 
