Holley sniper w/ headers troubleshoot

[DattMtn]

Hey all, have had a Holley sniper EFI system installed for about a year now on my 258 and it was running pretty good. My cast headers cracked and I replaced it with Hedman Headers (I do not suggest these headers, they do not clear the engine mounts). At the same time I upgraded to the 2bbl BBD cast intake as my motor came with the 1bbl. After some headaches, bashing, and grinding, I got both headers installed and now the engine is running like trash. I have reset the Holley Sniper to relearn the base fuel map etc etc and still need to drive it a bit to let that dialed in. It idles fine, actually better than it's ever idled, but my major complaint is that it runs the best on about 10-20% throttle, anything more and it stutters/spits or just falls on its face and doesn't have any power. If I hold it on 10-20% throttle it will accelerate way harder than WOT. I have tried to scope out a vacuum leak by spraying down suspect areas with starter fluid and haven't found anywhere it revs up due to pulling in air. Right now I am running the factory air cleaner (long snout style) but will be replacing it with a round type, replacing valve cover breathers and PCV's etc.

My leading suspects are exhaust/intake leaks, it's always an issue with what you just worked on right? But what are other ways to find either? I have a mechanical fan on the front and I am considering pulling the belt so I have still air to feel for puffs (running it for very short times).

[babywag]

Not nearly enough information.
What is the AFR being displayed when it's running poorly?

Since it ran fine before...I would look closer for vacuum and exhaust leaks.

Also all the usual suspects, proper wiring & connections.
An exhaust leak will tell the ecm it's too lean and dump too much fuel resulting in it running like poo.
A faulty CTS will tell the ecm that the engine is way colder/hotter than it is resulting in the incorrect AFR and running like poo.

Everything has to be correct especially for a "self-learning" lmao system.
If WBO2 data is incorrect it WILL run like .

[Crankyolman]

To find exhaust leaks I take a piece of hose, hold one end to my ear, the other I fish around suspected areas of leakage. When you find the leak you can hear the difference and often feel the hot air blowing through the hose.