I'd bet the car still runs rich in any time that's not a performance mode. A lot of places won't be good at tuning for this since it usually difficult to do. My experience is that the "rules of thumb" regarding powervalves and jetting are wrong a lot. I went EFI because I could never get it exactly where I want it, and its a lot easier now to get it right. I had to spend a couple days after I got it back from the dyno when the engine was built to have it drive right.

I'll bet the plugs are fouled, the car should get at least 10 MPG. I can beat the crap out of my 340, sit in traffic at the dream cruise, do a lot of short trips, and still get 13 mpg with a 4-speed 3.23's and going 75 mph (3300 rpm) on the freeway, and it only makes maybe 50 less HP than you have and even wider tires. When I had the carb it would get about 11.5 at worst.

If they are forged pistons and its cooler/cold out they will slap a little.