Fuel pump push rod length? Replaced the rubber fuel line sections from the tank forward?
The OEM-style fuel pump is very efficient at lower rpms, but allegedly becomes less so at higher rpms. A shorter total stroke due to the worn push rod can possibly make it look "good" at lower rpms, but less so at higher rpms?
What road speed are you doing the WOT-kickdown at? What happens when doing kickdown at 65mph? Or if you do a progressive throttle increase without getting into kickdown? If those "progressive applicatiion short of WOT" happen reasonably well, then the metering rods in the carb are still moving up and down. A slightly-flat response (after any accel pump shot is consumed) would mean the carb mixture needs some more enrichment (primary jets/rods), but there should not be any backfires or similar.
On my '67 Newport 383 4bbl, the OEM AFB is a full manual-secondary carb. NO air valve in the secondary side, but from my experiencies, the total size of the carb is not too great to not tolerate WOT from idle (with the stock 3.23/H78-14 factory tire size equivalent). When I dressed-down the casting flash on the venturis and finessed the gasket matches between the carb sections, then I to exercise a little bit more discretion and use a slightly higher rpm (than base idle) for WOT applications. With the factory cam.
Get a quality carb kit for the carb. With the carb off of the engine, remove the carb's top casting, which should expose both of the float bowls. The secondary jets are the ones at the back of the respective float bowl areas. With proper safety shielding of your eyes and such, carefully apply some spray carb cleaner to the jets (with the straw on the aerosol carb cleaner's nozzle) to see if fuel comes to the secondary venturis, on each respective side. The fuel feed circuits should be shown in the Chrylser service manual.
With the modern AFB/AVS (or post '67 OEM 4bbl), youi'll need a new air cleaner base plate with the larger "Holley-size" (for back then) hole in it.
Before the engine rebuild, was this performance issue operative?
Just curious,
CBODY67