Well, that is easy.
In the "Window Settings" Dialog, go to the "Floor Plan and Section" panel, make sure the "Floor Plan Display field is set to "Symbolic".
Then check the "Override Object's Line Types" checkbox and specify your Line Type in the "Symbol Line" field.
If the "Floor Plan Display field is set to "Projected" or "Projected with Overhead" or "Overhead All", then you will have "Uncut Lines" and/or "Overhead Lines" fields (you will not be able to override the Cut Lines then).
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