GTO HOOD TACH???

Hey guys,
I need some help. I figured one of y'all might know.. I'm restoring a 69' GTO hood tach to use on my 81' El Camino. I'm also using a his & hers shifter from a GTO.. GTO's had some b!tchin options..... Anyways, I need to figure out how to hook it up to my 81'.. I already have the rally factory tach and it uses a tach filter system. My 81' will soon be computer delete so I'll have a standard HEI dist system. The hood tach still has the factory harness, I need to know which wires will need to go where. Thanks for any help. Someday I'll have a 66' GTO to wrench on, until then I'll piddle with the later model GM's...

Maybe the Buick GS or Firebird guys might have a clue about these hood tachs...