LS 69 camaro

hi i have a 68 camaro that i am looking at dropping a 2003 5.3 and 4l80e.i would like to up the compression some and change out the stock cam for something more performance suited.i am not looking at setting the world on fire for power.just looking to make a few more ponys and still get good mileage.i am looking for great street manners with as much gas mileage going down the highway.i would like to get somewhere between 15 and 25mpg while cruze is set at 75 or so.this will be efi and stock pcm.not sure on rear gear yet.i will also be using a truck manifold.any help on cam,compression,and rear gears would be great.what can a person expect to get for mpg in a 3200lb car.thanks in advance