Don't use those things, they're a hack and a gimmick. If you need a bidirectional bus, use control signals to set data direction (e.g. most old-school parallel buses), or double the number of lines and use fixed transmitters and receivers.
6 inches is not a "long line", I don't have any problem with running 3.3V logic over that, board-to-board, as long as, for example, ribbon cable with alternating grounds is used (as mentioned above). Given some limitations of course, but that shouldn't be terrible for commercial purposes anyway.
If the risetimes are sharp (a few ns or less), use source or load termination resistors, or both; if both, consider a line receiver like 74HC7014, or anything comparable. (You may need comparators to get a reliable logic input threshold, but in that case, consider LVDS receivers -- they are quite capable RRI comparators in their own right, and cheaper than the fully-spec'd alternative).
Tim