slight pop when let off accelerator

Hello all, thought I could fix it on my own.. but need some guidance. I have an 87 speed density 5.0. All stock save a K and N panel filter, PVC tube to replace the stock rubber intake tube, and flows with dumps right at the axles. I have a pop out the exhaust when letting off the accelerator (revving or shifting) after revving. It happens under load and with no load - little more noticeable when cold but happens in all weather
* moved timing around from 10 to 18 with no real change.
* Changed ACT sensor and Coolant temp sensor, changed O2 sensors, better throttle response but no change to popping
* new plugs, wires, cap, rotor - still popping
* swapped out intake for an extrude honed stocker (kept stock EGR spacer and TB) I had around (wanted to see what the gaskets looked like anyway) the rear intake rail gasket had folded in so I thought I finally located the culprit. Put it all back together and still pops.... haven't really touched TPS or IAC yet.... Any thoughts?

one other thing I thought of.... swapped out the old 87 19# injectors with some 19# I had from my '95 sitting around while I had the intake swap going on

backfire.. not the classic "duck, someone is shooting" backfire.. more of a slight pop - backfire

I did change out the fuel filter too....