1998 Cadillac Catera removed the heads on to replace leaking head gaskets

We are working on a 1998 Catera that we removed the heads on to replace leaking head gaskets. When we put it back together we had a mechanic come out from Cadillac to set up the timing belt. At that time he checked the cam gears and made sure that the cams were in the right location. The intake cams were coded with a "G" and the exhaust cams were coded with an "A." The cam gears were properly installed with the timing marks used at 1, 2, 3 and 4 from passenger side to drivers side while looking towards engine from harmonic balancer. The engine lacks power, backfires through the intake, sets codes P0302, P0304, and P0306 all at the same time. One time it even set a code P0336 which is a crank sensor code. We have checked the spark on cylinders 2, 4 and 6. Spark seems fine. We have removed the plugs and 1, 3 and 5 are firing fine. Plugs 2, 4 and 6 seem to be wet. We have checked all the connections and found them to be OK. We are missing something and we cannot figure it out. We also had the man back from Cadillac and he is also stumped. Please help