Somebody's blowing smoke up your exhaust. You can put duals on any engine, but there are limitations. The problem comes with too much pipe. The engine produces higher torque at low speed with some back-pressure on the exhaust side. If the pipe gets too big, you lose the bottom end torque, which is what gets your truck moving.
My recommendation on dual size is 2" for a stock or mild performance 350. Step up to 2-1/4" over 325 hp, and go up to 2-1/2" above 375 hp. I will admit that these numbers are on the conservative side, but they do take into account the fact that unless you are racing, you rarely have the engine at high enough rpm to need the extra capacity.
