Nice to see another Fiero guy from Europe.
I have an 88GT and I completely re-did my interior last year
The big problem I came across was the speed sensor.
The signal from the speed sensor goes to the speedo first BEFORE going to the computer. The original Fiero speedo then halfs the frequency before it is sent to the computer. If you remove that speedometer and install a new one, then no signal will go to the computer and the car runs like you put sugar in the tank. I eventually solved this problem by installing the original speedo gauge in parralel with the new one and hiding it inside the new dashboard.
I am sure someone else on here can explain that better than me.
I look forward to seeing some photos of your work.