What could have happened?????

Oy. OK I changed the head gasket and intake/exhaust gasket today on my Jeep 4.0. It fired up fine but I seemed to be smelling/seeing exhaust up near the engine (By this time it was completely dark) But the engine was idling fine as if all 6 cylinders were fine..OK good. I was happy. I drove around the block to the gas station and filled up and it drove just fine...After adding some gas it started up but when I put it in drive and gave it gas it started struggling and sputtering along as if it was starving for fuel. I made it back up to the lot and found that if I drove in reverse or just reved the engine in park it ran perfectly but as soon as I pout it in drive and gave it gas it would start the struggling. I tried to make it home but the Jeep would go from running ok to sputtering again. I finally had to stop at a red light at the bottom of a big hill and it couldn`t make it up the hill..I rolled back down and then found it would stall out as soon as I started it...I finally got it to stay running after 5-6 attempts but even with the peddle to the floor it was only going like 3 MPH..I just pushed it off the road and left it there for the night..I am so upset and disappointed...Knowing all that had to be taken off and reinstalled for the cylinder head job can anyone thing what could possibly have gone wrong to result in the syptoms I`ve described? I did all the gaskets, torquing patterns, everything back where it started by the book. There are no error codes showing up :-(