If someone posts a wide PRE block or a large image file, the entire thread widens to the width of that block or image. I wish the cells in the table that make up the thread could have independent widths so that one wide post didn't screw up the rest of the thread.

Sounds like you read in flat mode, like me. The way to fix the wide post problem is to make each post its own table rather than its own row. Some message boards do this, but I can't recall off-hand where I've seen it like that.
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--The Amigo