If you want to get the best MPGs I would go EFI. If the bolt pattern is similar to a later model 90s GM V8 I would pull the Lower and Upper Intake manifolds from a junk yard along with the fuel rails, injectors, MAP sensor(should be in the Upper intake), Throttle Body (with all sensors, the PCM and its wiring harness, then buy a new O2 sensor. Depending on where the injectors run to you might need heads also (if injectors run to the heads instead of lower intake).
Have a local tuner delete the missing codes out your are in Chicago so that should not be a problem look at
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You will just need to research what parts will bolt on to your block and what year PCMs can be edited.