Fuel Efficient LS Engine

[Xedlos]

So I got a new job and Im now driving about 30 miles one way so 60 miles round trip and its all highway. I currently drive a 99 olds aurora and I get an average of 20MPG. Im ok with the fuel economy I am getting.
I want to drive a pre 75 car as a daily driver. So Im bouncing around the idea of purchasing a car and dropping in a fuel injected engine like an LS engine.

Ive been reading all over and I keep reading stories of people having some crazy fuel efficient stories from LS engines. I know that fuel efficiency will also be influenced by car aero dynamics trans and rear end.

What would be a good ls engine that can get me at least 20 MPG on highway. I dont care much for horsepower or torque im more interested in fuel efficiency.

[srh3trinity]

If you stay mild with the cam and gearing and go with a t56 or 6l80 you could get 20 mpg mixed driving. I drive about 90 miles every day and about 80 of it is interstate. I average 23.5-24 consistently in a g8 with a 6.0l and a 6l80e.

[hotrodneb]

I drive alot for work...put 200,000 on a stock '02 Z-28. I never really tracked the mileage but was always over 20, all highway driving.

[BuzzKillian]

I have a LS/1 T56 in a 68 Camaro. (all stock) The only time it drops below 20 MPG is when I'm AutoXing or on a Road Course. I got 26.6 MPG going to Motorstate. (250 miles) I drive 40 miles round trip to work and it's always 20-22 MPG.
I check almost every tank. (even when I'm racing it)

[garys 68]

I did the exact same thing. 72 Olds with a 5.3 LM7, 4 speed T10, 2.73 rear. Even without OD, I'm over 20mpg. The 5.3 iron block truck motors go for 1/4 the price of the aluminum block LS motors.