Changing from idiot lights to rally gauges
 

I have a 75 GP with idiot lights and would like to change over to the rally gauges including the tach. The 75 Pontiac service manual only shows 1 printed circuit board in the pull out wiring diagrams. I also bought what I seem to remember is a complete NOS dash wiring harness years ago on EBay but it was listed for a 74. Is it necessary to replace the entire wiring harness? I realize I have to change out the sending units for oil pressure and coolant temperature. Wiring doesn’t intimidate me in the least. Any tips or pointers would be greatly appreciated.