1. Replacing exhaust manifolds with headers. This probably will give me very noticeable boost in HP and maybe bit better mpg since I already have aftermarket exhaust that is restricted by the manifolds. Headers should also weight less than manifolds.
Yes headers will improve your horsepower by improving the volumetric efficiency of the motor. It does this by allowing you to burn even more gas by getting rid of exhaust gasses that were still trapped in the cylinder when the exhaust valve closes. A header makes the motor look bigger to the carb as it can breathe in more fresh air. Burning more gas is not the secret to better gas mileage.
2. Adding cold air intake. Not sure how because I do not know if anything like that exists for carbed 350s but I guess I can build something.
Yes colder air is denser air. Denser air contains more oxygen molecules to burn the gas more completely. This is free horsepower and can improve your mileage as well as no additional gas is entering the system.
3. Switching 2bbl carb to Rochester QuadraJet that will give me smaller primaries that may save gas. I just have to check the prices as it will probably be expensive job (intake, carb, etc).
This is a positive move forward and can be a win/win situation. But only if you keep your foot out of it. You will be tempted, a lot! I would recommend an old valve spring stuck under the gas pedal if you are serious about mileage
5. Replacing front drum brakes with discs. Not sure if I will drop any weight here but it will definitely improve braking performance.
Won't help mileage in fact it actually reduces mileage a little as the disc brake pads are always just touching the disc ready to be applied with force. It is mandated by safety plain and simple.
The one thing you didn't list helping mileage you want to take off and that is the cruise control. You would be amazed at how driving a one constant speed (say the speed limit) saves on gas and on tickets. Don't forget your car doesn't look like a bath tub upside down made in Eurasia, so you attract the eyes of the police as well as car thieves (never give out your address at a car show to a stranger, cell phone numbers are fine, as are e-mails).
Google fiberglass hood for manufactures and then check their catalog for parts. Might surprise you what you will find available. Generally that type glass is for racing so it is quite light weight (many are for lift off only).
If you are serious about mileage I would recommend either a late model Generation II small block LT-1 out of an Impala SS or a LT-4 out of a Corvette as a transplant. The EFI will yield high mileage and provide more power than a carb could with pump gas as it changes from tank to tank and the computer stays on top of the combustion process. If you still want a modicum of power and extreme mileage get an EcoTec four cylinder transplant as with turbo charging can make as much power as you get out of your V-8 but with much better mileage.