I replaced my idiot light dash with a guage dash, but I also had to replace my interior harness, so I just bought the guage harness. Both the tech tips section at the main page of this board, and GMCPauls site show how to re-wire the plug to swap the dashes. Paul's shows the pinout, etc..
http://www.gmcpaulstruckparts.com/Tech%20Tips1.htm
My under hood harnesses were also bad, so I went ahead and bought those for a guage dash, so that wasn't as bad for me.
My biggest problem has been finding the stock oil pressure line and fitting, but the board members should be able to help out with that (thanks JUNKYARDJOHN!).
I don't know about the aftermarket tach or the fiber optic.
Good luck!
Mike