Couple of things.
First, your new free-breathing exhaust has now leaned out your fuel/air mixtures. You may need to re-jet.
An air pump equipped Bronco may already have been running leaner jetting, but one that's a '73 or later would be even more so. What year is your Bronco?
Two, do you know that it's the original engine? And even if so, do you know whether the cam has ever been changed? The ignition firing order follows the cam, and there are two different versions commonly available for the 302/5.0 engines.
Your original cam would definitely have the early 15426378 order. Unless it's a 351 by some chance?
But a cam change can alter the firing order on any Windsor-family engine. So when in doubt (such as not knowing the history of the engine), checking is needed.
Three would be something wrong with the plug wires. Not just the firing order, but perhaps a bad wire. This is not common, but it's not unheard of either. And anything that's possible can always happen with any of us.
A lean mixture "usually" would end up with popping on deceleration. But a hyper-lean mixture can pop under accel too.
A misfire from a single bad plug wire might not cause this, but coupled with your new leaner mixture (it is for sure leaner than it was before, whether it's the cause of your issue or not) could do it.
Multiple bad wires, coupled with anything else could do it too.
A question might be how does it idle? If it's perfectly smooth and transitions from idle to accelerate easily and cleanly, it's likely not a bad plug wire or out of order wiring. But it's still possible.
Good luck.
Paul
. I put in new 8.5mm spark plug wires when I had the kit installed.
