L67 /4T65e HD is going to be the most cost effective too in the end.
You start bone stock at 242 HP and just doing the basics in the PCM will see over 250hp without even doing a "power tune".
City will be about 15-17 mpg, highway will see just under 30 mpg.
All in all not bad considering you'll be able to do 0-60 MPH in 6.5 seconds with a stock engine and tranny from the wrecking yard.