Well, a little further along. The barometer module arrived, so I managed to get it working to give the baseline atmospheric pressure to zero the gauge. This means that the gauge should always give the right boost figures at any elevation.
I test fitted it into a spare tach I have. I dropped the OLED screen onto the floor and cracked it, these are really delicate. The screen is now only showing what looks like every second pixel row, so not as sharp as it shoiuld be.
Once my DC-DC adapter gets here, and I get a replacement screen, I will fit this tach into the car and see how it works. My main concern is how bright it may be at night, I need to see if it is possible to dim these screens, and get it to read the light switch. [Edit] No, these particular monochrome screens I am using cannot alter their brightness, the OLEDs are either on or off. Maybe a different brand might be able to.