is the carb running rich? that will deff. kill gas mileage, i have the same engine with a 4 barrel 600 holley and i get about 17 mpg or so. check normal stuff, good plug / spark, clean air filter, etc.. but anyway when my carb was running rich i was getting about the same as you, just tuned it and it got WAY better.

 

Well I believe the power valve is blown and the jets need to be replaced so mabye that is part of the problem? I know it is running somewhat rich because I can smell it to some extent.