Newbie with 454 sucks 6 mpg

[chumstone]

Great site! I've had this 73 Chevy now for the last 7 years and every chevy truck I've owned has made me a chevy man.
OK the truck is a Camper special, and had 60k when I got it from an old guy who ordered it new. He said that it got 12 mpg when it was new, I got 11mpg highway, 9 mpg city. Well the thing started to smoke and I decided to get another engine. I did not know what I was getting into at the time, but I found a rebuilt 1985, 454, bored .30 over, precision balanced engine, and has 10 to 1 flattop pistons, RV cam, 1991 motorhome heads, double roller timing set, high volume oil pump, Edlebrock intake manifold, pertronix HEI and Edlebrock 1411 750 cfm carb. Single 3" exhaust and chrome carb aircleaner, paper element. Also Turbo 400, 2x4, 3.73 gears with utility body and truck weighs 8700. I'll add signature later.
The engine has lots of power and pulls real hard! Driving less than 1000 miles a year because of the low mpg. Anybody got any ideas to improve fuel economy? Should I have installed a small block 350? Anything that a couple of hundred would do to get 2 more mpg?