the rails are drilled and tapped for 8an straight oring ,
the feed is -8an male x -8 straight oring
the fitting for the rail for hooking up a gauge is a -8 straight oring x -6an , if i were going to replace i would try to get in -4an outlet so std gauge lines and senders can be connected but that might not be possible coming from a -8
you could get a -6 female to -4 male adapter and screwit on to allow gauge hookup