You said you had 56 psi fuel pressure. Are you saying that is not the case?
Ignition concerns like moisture in the cap, carbon tracks on spark plugs, distributor timing( cam offset). these things when under a loaded condition will cause a cylinder misfire, low power and a backfire, when the spark plug starts to fire again, and there is a rich fuel mixture in the cylinder.
Connect your fuel pressure gauge up and drive it under the conditions of the low power and back fire. If it holds 55-60 psi, fuel pump is good. Fuel filters that are restricted can cause low power , but i have not had them cause a backfire.
#1 Look for a black carbon track, or a cracked spark plug insulator.
#2 inspect the spark plug boots for small pinholes. A small white burn witness mark on plug boot may be present if there is a hole in boot.
#3 Coil wire terminal connections.
#4 burn through marks in distributor cap.( look where two terminal come close together) this cap has terminals that cross the cap, and can jump through the cap material.
#5 P0174 rt bank lean fuel mixture. swap o2 sensors bank to bank and see if the lean condition follows the o2 sensor. ( it will set p0171) , if so it could be a skewed sensor and should be replaced. A sensor that creates a false lean condition , will cause the computer to richen up the injectors on that side. This can cause converter to overheat, and melt down, resricting exhaust flow. a pressure test can be performed on the cat to make sure it is not restricted.
These are the most common causes that I have run into . Just by changing plugs can cause the boot not to seal properly around the plug, This opens up the opportunity for moisture to get in and cause the spark to jump. Once it starts jumping to ground, it starts building a carbon track. The longer it goes on the more frequently it occurs.
If this is what you find, replace the spark plug and the plug wire, at the same time.
If you need more information, Please Ask.