What engine and transmission do you have in this Chevelle? For the times, both the six and small block V-8 were efficient as compared with other engines. If you have the 2 speed PowerGlide, the gas mileage around town won’t be great.
I would tune up the engine with the parts specified by the factory–no fancy mufflers, air filters, etc. Non-factory specified parts probably won’t make that much difference in the gasoline mileage and you would be a very long time paying for these non-standard parts for any difference in gasoline mileage.
Get the car running the way the factory intended and enjoy the ride!