I've been thinking of doing the same thing with my 2000 Blazer and putting it in my 2002 S10. Things to note here...schematics. You will need them. Alldatadiy.com
Fuel mileage is based off of fuel level data from your engine ECU. Not sure if your 98 ECU supports this, but either way, you will have to tie this data line into one of the data lines on the OBD2 harness. This is how your trip mileage, MPG etc is calculated. Compass is built in. Outside air temp, you will need the temp sensor from the front of the truck(I don't remember where its mounted) and you need to supply it with 5V from the ECU, then from the sensor to the computer on the roof.