Stick to the fuel injection. There is a pretty significant mileage advantage using fuel injection over a carb in an application like yours. Stick with the factory wiring harness and computer if you can.
Now if your main goal was drag racing then my answer would be different, but you want real high mileage and a quadrajet isn't the way to do it.
E85 racer and E85 carb builder
68 Camaro 427" E85 powered small block, 9.95 @ 133 mph best motor ET through the mufflers. 1.319 best sixty foot.