Punching it at 1500 rpm with a stock cam can cause lugging and bad firing.  Not necessarily your problem.  Basically, you are too low on the power curve.

What is your rear end ratio?  One of the 2.79-2.80 rear ends will make the problem worse.

I would verify resistance of all plug wires and verify they are going to correct location.

Did you verify camshaft firing order?  Since it is an unknown. Possibly?
Even a 289 firing order will run but idle is affected…look up Mustangs Plus efi swap.

Did you perform the engine balance test?
That may help locate which cylinders to look at first.

If EGR is removed you really should have a resistor bridge in the circuitry to fake out the ECM.  But I don't believe the lack there of will cause it to pop back through the TB.  I have had this happen a couple of times at cold startup while I was sorting out my last foolishness.  What I found was a bad Hall pick-up coil in the dizzy.  I discovered that after about an hour heat soak, the ignition went to crap and though it would run, it was just barely running but I don't recall any backfiring other than during startup. 
All I can suggest is that you systematically trouble shoot the fuel and ignition systems and make dang sure there are no vacuum leaks.  I'd start with the vac leaks.  I plugged every vac line including the line to the PCV.  Made no difference.  Then I checked the fuel pressure.  Right on.  Then started on the ignition.  Last thing was the Hall pickup since the dizzy needs to be pressed apart to replace it.  That was the culprit.
When I was fooling with cams/injectors/etc....making a hot rod....I installed an AFR gauge.  Best tool on the car for seeing if the EFI system is working correctly.

Oh...PM me your email addy and I'll shoot you the schematic for a simple three resistor bridge for the ECR removal.

Last edited by Bullet Bob (7/07/2022 6:59 AM)

Well i redid the resistor plug for the egr eliminator, took it for a test drive and no backfire. Tried quite a few times and it never backfired once. Had a stumble once, before it warmed up, but that was it.
Before i drove it, i tested all the plug wires, checked the firing order and unplugged the battery for an hour. Ran a koeo test and got codes 67-81-82-85-84. it cycled through those again and then 11-51
After i got back i got the same codes with 11-33
 
The first time i made this i just soldered the resistors one to each pin and hooked them together at the top.

I am assuming the ecm was changing the timing to adjust for exhaust gas reciculating and causing the backfire. Just a theory.

Now its cocktail time

 

Last edited by Alan (7/07/2022 8:28 PM)