A properly dialed in carburetor, and by that i mean using an AFR gauge to jet/meter it for your driving situation, can approach an EFI in terms of fuel efficiency. It won't likely match EFI because it isn't a closed-loop system but it can be very good. Same with Electronic ignition. When running right, the old systems worked extremely well - modern electronics brought better starting, reliability, less maintenance, and lower emissions for the most part.
In short, I don't think you will gain that much fuel economy by putting modern electronics on a vintage engine unless your current carb is out of whack and you don't want to tune it.
