but I'm trying to convert my gas 90 Jetta coupe into a diesel and don't want to hack it up too much.
That's why I bought the cluster I have as well... but ended up finding an A3.
As far as I know the cluster should be plug and play... the only thing your gasser harness may not be pre-wired for is the glow plug LED:
T28/20 Glow Plug Indicator U1/08 Green/White
Pin 20, Green/White wire.
Other than that it looks like everything else, including the tach, is exactly the same between engines.