4L80e problems

[Ctinman]

I have bee doing a 6.0 LQ4/4l80 swap on my 70 c10. This is my first ls swap so it's been a huge learning experience for me. Anyway, I now have the truck running again and went to drive it and the 4l80 is acting up. It goes into reverse and drive fine but when you start moving it shifts very quickly(as in it is shifts into second when you are just idleing along) and when it does the engine tries to stall. I haven't driven it very far but I'm pretty sure it isn't shifting past second either. And if you put the shift lever down in first, it starts in first but still shifts to second very quickly. A little background, it is a freshly rebuilt tranny, I have filled it to the proper level, and it has no obvious leaks. I purchased it from the same vendor I purchased the engine, and wiring harness/pcm from. The pcm was supposed to already be programmed to run this engine and tranny. The check engine light came on and I had a P0740 code. From everything I see when I search this code, it looks like this should not be active with a 4L80e. That seems to me like it should be impossible to get this code if the computer is programmed correctly. Is this correct? Any help would be greatly appreciated, I'm just trying to figure out what my next step should be in figuring out the issue.