If the white wire is the speed signal (can't remember from memory), then you can't use the speed-related features, such as gear-based boost etc. Not essential though.

On the RST, we can use an adaptor from an EFi car (or one with a fuel computer) which screws inline with the speedo cable and give a signal output. Not sure what you can do on the Cossie though. There must be a similar way.