you see, even your three middle guages have the plastic cylinder that recesses the guage farther back, if you want your guage recessed then you'd find a way to make that work, if you removed the plastic cylinder then your guage would be more flush to your dash and you'd configure another way of doing it. that's why i couldn't be like "BOOM this is the way you mount an aftermarket guage into your Z....blah blah blah" because everyone likes it the way they like it, and no matter how you do it, there's a couple ways you can do it. LOL

i'm not sure of the little sizes but the typical autometer boost guage you see at any parts store is the right size, the easiest way is usually taking out the head unit and air conditioning control head then reaching in and up. on my center 3 guages there's a piece of metal strip that wedges in at the top then has 2 phillips head screws that hold it at the bottom, just take the 2 screws out and flip the metal strip up and the guage should pop out. then just unplug the old one and figure out the best way to mount your new one in there. some people use epoxy, some people customize the metal strip some people make their own bracket, it's really just whatever works for u. when in doubt just match up as close as you can your new guage to the old guage.