What kind of gas mileage would a 1968 El Camino get with 396 cc engine?

A 1968 El Camino with a 396 cubic inch (not cc) engine would get very poor gas mileage by today's standards. You're likely looking at somewhere in the range of 8-12 miles per gallon (mpg), and possibly even less depending on driving habits, vehicle condition, and the specific carburetor and other engine modifications. Highway driving might yield slightly better results than city driving.