You will never realize a return on your investment(time and/or $$).

It will still require about the same amount of energy to move your
vehicle, save for the modest weight reduction.

If you need to go to such an extreme, go straight for the 240 or 300 I-6
with a 1 bbl. carb.  You may reach 20/21 mpg highway with the
long-stroke six.  17 is a reasonable MPG expectation, though.

Rob

I have a 66 GT A-code with a slightly modified 289 (Eddelbrock Intake, Holly
570 carb).  It has auto trans and 3.00 gears in the rear and I am seeing
about 16.5 mpg combined city/highway driving.  I did notice if I cruised the
freeway at 80 mph I was only getting 13 mpg.  I tried dropping my speed down
to 70 and my mileage shot up to 16.5.  Im sure if I were to swap the stock 3
speed for an AOD I would see an even better improvement.

    Will you get 20mpg with a 302?  Maybe.  But you
have to be very carefull on how you build it.  Mild
cam, EFI, electronic ignition.   Don't get to carried
away here, or any savings you might make will be eaten
up by fancy parts.
    Your best bet would be a late model 5.0 from a
Mustang.  That way you get the proper computer and a T5
trans.  I'd almost bet there's off the shelf parts
available for this swap.
    As someone else mentioned, look in other areas for
savings, too.  Reduce weight.  Skinny tires.  Better
aerodynamics.  Driving style.
    Do the math.  What mileage do you get now?  What
will you spend to do the swap?  How many miles will it
take to recoup the expenditures?
    Of course, a 5.0/T5 in a '66 pickup would be a lot
of fun to drive. So don't forget the fun facter; that's
got to be worth something :)