Does the 1999 6.0 need anything specific to hook up to a 4L80E?

yes if it is the extended crank version you need to use the flat 6.0 LS flywheel with a special converter that is made for that setup...I've been through this road already...
 
I don’t know the measurement but the main thing is that you measure converter pullout and it is in spec with what your converter Company reccommends.
 
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