How do I take out my old temp/oil pressure gauge out and replace with the new one on my 75' 280Z?

to remove your glove compartment and go threw there.

You will need a short cross-point screw driver, ~3" long and bandaids:ermm:

On 240's you go through the glove comp, but late 260 and 280's, you unscrew 4 screws that hold the center vent assmbly in the dash. then 2 brackets that it attaches to (2 screws each) remove, then pull out the plastic air vent in back (slide the hose on each side off & wiggle it out).

then use a short handle screwdriver to unhook the strap that holds the gauge in place. the top of the strap will pivot so you can pull the gauge out, you just have to unplug the wiring- thats it- i've done it five times in eight months in my 260- i am getting mighty quick now!:cross-eye