dual fuel tank gauge not working

[bakerboys]

have 85 Chevy K20 4x4
was a diesel now 350 has dual tanks both do work but gauge will not work
my gauge stays om past full all the time
hooked up another gauge panel from junkyard and fuel gauge went straight past full mark

Checked all grounds seem to be hooked up and grounded

I dropped right side tank because it leaks
noticed tank on driver side seems to be wired differently

Any thing I can LOOK for thanks