1997 vortex 7.4 fresh engine open exhaust JBA shorty header and no cats problem

[Suspect Racing]

The trucks pulls like a bull in open loop. When it switches to closed loop it falls on it?s face and struggles to just drive part throttle. 2.5? exhaust through an x pipe to the rear axle then through a flow master two into one then tailpipe. Fresh engine with balance job tight build and reground stock cam. New everything new sensors, fresh ignition knock sensors everything no codes. Symptoms surging and misfire only in closed loop. What is the computer seeing that makes this happen? With the open exhaust do I need a new tune?

[RDF1]

It would help if we could see what the computer sees as well.. The tune helps a lot too.