Hi. I have 68 with 5.0 HO engine AOD tranny 2 barrel carb autolite and was wondering if it will a good swap to 4 barrel holley 600 with ford 4 intake Mann. My rear gears are 279. I'm not going to be racing or anything just lookng for a little mpg saving . Thanks

 

Use a 500 if you want to improve mileage. And have your distributor advance adjusted in a distributor machine.

 

Ok. So can I use the stock 4v intake and 500 or less? I have a H.E.I. Type distributor

 

Now is your goal to have improved perfomance without being a gas guzzler? Or is your goal to improve MPG? Or is there something wrong with your 2100?

The basics need to be covered tire pressure, new plugs, ensuring your brakes arent draging, etc. All of those things that every car need to get max MPG.
Changing your driving habits to stay out of the throttle.

If you want better performance with out going crazy on gas. Then I would say get your hands on a autolite 4100. This is carb flows about 500 CFM, is stock so there shouldnt be alot modification work and it is the simplest most reliable carb that you an put on it. Unless you have built the engine to need the higher CFM that aftermarket carbs provide. The 4100 will be your easiest option.

How ever if you just want better MPG stick with the 2100. As soon as you open your secondaries of a 4v then your MPG is shot.

 

An AOD and 2.79 rear gears is keeping your carburetor in the idle and off-idle circuits even at 60 mph. These are very rich which lower mileage. The engine rpm are so low that anytime you press on the gas pedal you are not only pumping fuel through the accelerator pump but the power valve is also opening.
Changing to a larger carb will make it worse. You need to change the rear end gears to near 4:1 (4.11:1 would be close to perfect. The best nod would be to change to EFI because then the AOD will work with the gears you have - EFI reads road speed and will lean the mixture to give you the mileage you are looking for.