RACEON

There have been many write ups and answers to the LT1 split blm issues.
So, if you have definite answers and actual fixes for split blm in regards to vacuum leaks etc.
My particular LT1 has a medium size camshaft, with 230 intake and 236 exhaust on a 114 LSA in a stroker. And I use 42# GM injectors slid Green BOSCH III's.
It has the modified AS&M throttle body to allow the air for idle to be sent through the proper idle circuitry.
The manifold has been port matched to heads.
The heads are ported and polished and I use good header gaskets etc.
I also have converted to heated 3 wire O2 sensors and my O2's are far upstream on thre header collector to not even be affected by a collector leak, if there was one. The wires are a ground, a power source to heat them and the actual sensor wire which has been altered and the sensors are good and tight DENSO sensors.
My manifold has a catch can plumbed into the port for PCV valve and back in to the factory route.
Intake is a 94-96 style with proper fuel rail and EGR and AIR blocked off.
So, where is the vacuum leak causing the split BLMs? They arent little splits, they are 20-30 off, with the left pulling fuel.
I have always had split BLMs in this car. I have had it with different manifold, headers, injectors, throttle bodies, ignitions (Opti to DELTEQ), unheated sensors to heated O2's and the list goes on. So obviously I have missed something to fix the split BLMS. Sorry for so long of a thread.
Now, i am not a novice on this car or engines etc, so if you have dealt with this and have solid answers or avenues to check, please let me know.

95Blackhawk

I have a bigger cam than you with a smaller LSA and my intake is set up similar to yours and no problems with split BLM's so here are some thoughts:

Confirm how you know ALL idle air is going to the idle air plenum? I know you are not a newbie but took me a moment to figure it out myself. FYI, once I did this all my split BLM problems went away.

I know you changed intakes but idle air holes to some of the cylinders could be plugged. Easy fix but takes some work to get that intake off and check. I high pressure hosed mine when I had it off last.

I would have the injectors flowed. Forget they are "new" etc. as you are having problems and you are asking for solutions.

RACEON

Injectors were flowed. I have sheets for them. They are also 141.-14.5 ohms warm.
I found plug number seven to be soaked in oil.
I took the manifold off to see if it was ripped and allowing oil into the chamber. None. It was dry as a bone.
I took the header off to replace the gasket, no burn marks on the gaskets.
Replaced O2 while there.
I am going to replace the manifold gasket and then take the car to my shop and take the two valve springs off for that cylinder and replace the valve stem seals. I will do a before and after compression check.
If the seals are bad, and no oil leaks or shows up after a few minutes of idling, as it did after I replaced header gaskets/plugs and everything, then I will pursue head work as maybe the guides are bad.
My throttle body is modified to allow all idle air to go through the passageway gm intended to through the manifold. I may take the bottom plate off and see if anything looks plugged, and run compressed air through it nonetheless.