The 5/16 sending unit with no fuel return is for 2 barrel applications I believe, because 2 barrel fuel pumps don't have a return either.
The 69 Z/28 never used a return either, and that sending unit will be 3/8 feed. Same deal with L-78's, no return on those and is the same sending unit the Z/28 uses.
Return style sending units with a 3/8 feed line were used on Q-jet applications, like the L48 350's and L34 396/350hp engines.
The one I linked to for Bryon is the correct unit for a 69 Z/28 and L78 application.
