Cossie, EEC4 to start with..

Then there is the aftermarket stuff like Pectel, Autronic, Motec, Gotec, Omex, GEMS, Haltech, Emerald, DTA etc etc...

Lots of options!

The MFI on a 5th injector will be ok but not ideal. The benefit of EFI wll be more ignition, fuel and boost control and better drivability.