Fuel gauges can always go a bit spastic once you've disconnected the battery and even more so if you've messed with the cluster, it should just fix itself hopefully.
I assume you meant you forgot to remove the relay and clear the fuel system before changing the injectors, should've called
i got little hooks for pulling o-rings out
and you do literally live 3 minutes away.
Forget Volts, you don't need it, what you want is oil pressure and boost gauge if your only going to run 2 gauges.
Buy the new headunit and i can do the re-wire this weekend coming, i got rolls of coloured wire here to extend and fix all the hack job connections the last owner did and fix it properly.
Gauges wire to ignition power and the lighting circuit and earth, you don't want them wired off the headunit remote wire for ignition power, i can find proper wiring to splice into to wire them in once you got them.
Can i have those red trims if you ain't going to use them, i need them to make something.