Smaller carbs can be counterproductive to fuel economy - the engine actually has to rev higher to pull in the same amount of air, and thus has more friction to work against. With a 318, get a 450-550cfm carb, and you should be good. Jet it just this side of lean so you don't get stumbling and pinging, and you'll be just about right. You could also re-jet the carb that's on there - I don't know about 76's, but most earlier carbs were jetted on the rich side for smooth running and reduced combustion temps, with the side effect of poor MPG and sooty deposits.
Dual exhaust will help a little, if you can keep your foot out of it.
Lockup trannies will help by keeping revs a little lower. If you can, an overdrive lockup trans would be your best bet. I don't know what's available for carb'd power plants, but I assume there's an equivalent to Ford's AOD that uses the standard small-block bolt pattern and a throttle valve cable rather than computers to control it.
E-fan will definitely see you an improvement in MPG and power - it's a mod I've been thinking about myself.
Cam, stay close to stock.
Another thought: run the thinnest oil in your rear diff that you can get away with. Less friction = less power loss = more MPG.
Lastly: your driving style can greatly affect your MPG results. A vacuum gauge and a tach can help you optimize how you accelerate and what speeds to cruise at for the best MPG, as well as other techniques like driving ahead to avoid stopping and keeping your mass moving, etc.
22mpg is going to be very, very tough, and probably unattainable, but I believe 18 is definitely within the realm of possibility (as long as you're talking highway cruising rather than average).
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