I recommend breaking your approach into two areas. Let's say something like:
- Big expensive gains
- Small inexpesive incremental gains
Big expensive gains would include converting the car to a five-speed manual transmission. Rebuilding or overhauling the engine to keep the combustion chamber tight. Then move on to advanced modifications such as EFI and engine management systems.
Small incremental gains would include good wires/plugs, electronic ignition, synthetic oil, your tall skinny tires. And wax the hell out of the car to make it as slippery as possible.
Meanwhile:
- Re-arrange your commute to find the route with the fewest stop signs / traffic signals. Shift your start time so you follow after the heaviest commute period, but before parents/students are driving kids to school. Come home after the evening commute period. Try to optimise the daylight time so you don't use your lights.
- Install a vacuum gage on your A pillar and pay attention to it.
- While you're waiting to install EFI, convert your automatic choke to manual.
- Back into parking spaces and your driveway rather than nose-in. At home, park in your garage and install a block heater.