I just took that dash back and got one out of a 99 Monte with a 3100 and it's the same, reads empty. Plug in the original dash and it's 3/4.
I did subscribe to all data for the single vehicle subscription, looks like I'll need to do a 99 Monte as well. The interchange says they both match though.
Wiring diagram says the wire from the fuel level sensor goes to the connector at pin C-14. I need a diagram for the Monte to see if it's in the same location.