a stock ls ecu is not going to provide enough fuel for that motor. at WOT the stock ecu will over fuel a motor into the 12's AFR but with that b20 you might be in the mid 13's which will let it run pretty good at WOT...partial throttle is a different story tho..... that MIGHT be the problem

im sure the iat isnt helping the situation either. fix that to eliminate that as a possible issue...easy fix