"GLOWING RED" Wow this has never happened before
 

Man this sucks and I have no clue what's going on farely new to the Ls world, but took my transmission out to have it rebuilt. After I installed my trans back in and ststarted my car it would idle around 2000rpms, and my headers would start glowing red!! this has never happened before I herd that its running lean but I can't understand why. all I did was take the trans out it was running ok before I took it out it I even had it running with the trans out and never had this problem. I've also herd that it could be a vacuum leak but I've checked for cracks and leaks and didn't find anything... Can someone please help I'm going APE SHIT OVER THIS!!!

Throwing any codes?
It does sound like a vacuum leak.
O2 sensors plugged in?
Exhaust not blocked or cats not plugged (Wouldn't explain the high idle though)

when the key is on all I get is transmission codes but I have a 700r4 so I know they going to show but my friend said that the map sensor was not reading when it was running...🤷🏿dum♂️🤷🏿dum♂️

Did you figure it out?
WHile the trans was out is it possible the engine rocked enough to break something off the back of the intake creating a leak?