Which LS engine has the best horsepower to gas mileage ratio?

[luckyusaf]

I am building a Pro Touring 66 Chevelle and wanting to put in an LS9 or LSA engine. After doing some more research there are not a lot of classics out there with these engines. I am curious as to why? Is it the cost factor? I want 600+ horsepower but honestly my main concern is fuel economy/gas mileage as this build is for my daily driver. I know the new vettes and CTS-Vs get decent mileage considering the horsepower they are pushing. To better some up my question...Which LS engine has the best horsepower to fuel economy ratio?

66 Chevelle
LS ?
Bowler 6L90E
Schwartz Performance Chassis
Moser 12-Bolt
RideTech Adjustable Coilovers
Wilwood 14" Rotors/6-Piston
Forgeline ZX3P 20x12/19x10
BF Goodrich Rubber