I know this is a popular code for any transmission problems but I had to ask it anyways and tell you guys what its doing. I took it to trans shop and they said it needed a complete rebuild even though it was done not even a year ago by previous owner. It does have a new torque converter also. Here's the story
I just put motor in it and I drove it straight to exhaust shop open headers got me some 3" duals put on it with h pipe (hehe). And drove it back home and as I got to 55 it started slipping in 4th almost up to 5200rpms, so I stopped at gas station checked fluid and it was a tad over full but nothing that wouldn't come out the tube later right? So I drove home which was almost a mile or 2 after that stop and it would hard shift to second and while in second start slipping from 2500 or so up to that 5k again even hard in it no change, wouldnt even try to hit 3rd. So I decided id just put it in 2nd instead of drive and as long as its in 2nd gear I can hold that gear to red line.
Now y is that? Even if im in drive and holding the pedal to Max the gears, should it slip going into third and not in the middle of second.

 

Btw I know there is a stickey on this I just see that particular problem. Also I threw a tps at it because i didnt have a computer at the time to check code and new that was a fix for some of the symptoms.
Also stupid question. Would not having o2 sensors at all cause trans problems? Like a limp mode or something?? Cause I put a obd1 engine in and haven't tuned out the no crank sensor and o2s yet

 

I would get your tune and see if the issues continued before throwing more money into it. If it was okay before you swapped the motor it is likely the root of your issue. If the problems persists then have another shop look at it. You said the trans was rebuilt a year ago? I would check the bellhousing bolts and torque converter bolts for looseness. I would also check for any connectors that are unplugged, loose, or damaged since you had the engine out it is possible one was left unhooked, got damaged, or is simply loose. Another problem i have seen is that the harness that goes into the trans that you have to pull the pan to unplug(not sure what it is for) is a pain to plug in and is easy to not get secured all the way and if not plugged in will cause slippage. i have witnessed this happen in person btw.

Check the simple stuff first....

 

I took it to another shop there going to check it out. It worked the the 50-60 miles that I drove round trip to exhaust shop. And I did check bolts before I took it to. Him just in case. Also checked the plug as that was my first suspicion was that it may have came loose but I remembered I zip tied it after install.