cracked 47 super exhaust manifold

My old exhaust manifold had a crack so I found one on line that was still attached to the engine. He sent me the exhaust and intake still connected together just off the parts car. I installed it and it was fine for several outings and then it started to hiss, I found a small crack in this one that wasn't there when I installed it. I used new gaskets on the exhaust and intake and I know that the factory only used gaskets on the intake. What the heck am I doing wrong to cause this to happen?
Any response to help please.
Jerry