Dumps are small smooth cornered elbows. A piece of pipe on the back of the muffler that curves down.. hence the term "Dumps" because it dumps the exhaust out the muffler directed at the ground. It makes everything a lot louder, but it also makse exhaust stink up your car.
And yeah, if you have dumps, it WILL stink up your car, especially if the temperature control is set in the hot position. Exhaust will enter through the outside air duct. You dont even have to have the blower motor on, it still gets in. Try turning it to the cool setting that way its not pulling outside air in.
5.0's have stinky ass exhaust, especially if you get rid of your cats and maybe even the smog pump. The exhaust on my 92 makes my eyes burn, and I smell like exhaust after getting out of it. 5.0's need cats. My guess is the combustion isnt as efficient as newer cars. My 96 has blown out cats and it doesn't smell that bad at all.