do you still want to run the  mechanical gauge in the cab?   just    take the  electric  switch out of the  391's housing,   and  plumb the  original  'wet' line  into that port.     hopefully  there's enough   tubing  to  adjust  for a little different   angle.  
  
edit;  that  'square'  brass  fitting is   like a street 'el'    it'll have  male threads  going into the   filter head,  and  the  actual switch  will have  male threads  going into it.    Unscrew that  block  out of the  filter head.   'should'  have the same  threads  as the  flair/ olive   fitting  you'd   swap out of the  330's