Bronco is running funny?

Well i have a 1979 ford bronco full size with a 400 big block in it. And today i took it to the bank which is like 10 miles away and i used a 1/4 of a tank getting there jsut there and that is not normal also everytime i stop or let off the gas it dies (it is not because the idle is to low) also it is pushin out hella black out of my pipes not sure what is wrong any help?