I hate to rain on this parade, your cause is very noble. I think a 66 Malibu/Chevelle will be aero limited at highway speeds to 30mpg or less. The 2 things that really kill you that can't be modified are illustrated below. The front end is a bull dozer plow. The idea for efficient aero is to have as much of the air hitting the front of the car to go around the sides and top, and as gently as possible, get it headed that way. The Chevy front end is quite the blunt instrument.
The second major issue is the transition area at the top of the windscreen to the roof, if you look at the most aerodynamic cars being made today, this transition no longer exists, it is just a smooth arch.
The real big issue is that the air will be so disrupted by these 2 areas it will never have a chance to smooth out and take advantage of any aerodynamic enhancements made to the rear of your car. It's pretty much a brick.
I think at moderate speeds and driving around town, you'll get way better mileage with the new engine, at 40-45 you'll get maybe 34mpg give er take, but as you get much over 45-50mph you're MPG will start to go down.
Again, I just wanted you to understand why we say the aerodynamics is a fly in the ointment for your cause, I hope I didn't offend you here.