I had the same problem when I got my car running. I got my LS7 from Stielow; it wasn't a crate motor, so it didn't come with that short pigtail harness. I didn't even realize that it was missing. I just plugged the engine harness in and it did exactly what you're saying - turn over for 6 to 8 seconds, then fire just fine. It threw a code, no cam pulse. I had a friend hook it up to a Tech 2, and sure enough, there was no signal (IIRC, it was high all the time). After looking at schematics and realizing that I didn't have that harness, the problem was found. For whatever reason, pins A and C are swapped within that 3 conductor harness that goes from the cam sensor to the oil pan rail. I have no idea why GM would have done that, but I simply used a Molex tool and swapped pins A and C at the cam sensor connector and all is good.
