Black Smoke

I have a 1998 chevy Z71 with 180,000 miles, the problem that I am having is that when I am on the highway and punch the gas, black smoke and soot come out of my exhaust.

i have tried several things from changing the sparkplug wires and plugs to trying various off the shelf products that claim to help stop this and clean your fuel system. I have also had different mechanics look at it and have not been able to fix my problem. On top of the smoke my gas mileage is really bad, with a 25 gallon tank, im lucky to get 180 miles out of it.

i'm at the point that I am contemplating replacing the engine, thinking this will save me money than continuing to have mechanics look at it. Is there any one else that has found a good soulution.