school me on dash's please

ok... well i have a 74 duster, with a old 120 mph dash... i have 4 rebuildable rallye dashs and im gonna have one worked over and replace my 120. I know what to expect price wise ( GULP ) but what about installation? i know the extension pieces will be needed, ( like to extend over the glove compartment) but what can i expect for wiring? will i need a new harness or will my old one work? i know ill need some more wires anyways for the gauges that arent included on the 120's... thanks a lot! I would be lost without fabo ...
:munky2: