When un-plug and plug any electrical connector always be cautioned to switch the ignition OFF.
Did you unplug the MAF while the car was running? If this is the case your ECU is going nut trying to map. You should erase all the codes and start over again and see what codes are going to come back.
If your CAT is original and with 259K miles I bet it's all clogged up, and this is the reason why you have back fired through the intake.
If I were you, it's time to shop for new CAT.
Replace the intake manifold gasket and all the hoses under it.
Replace the fuel pressure regulator.