backfiring problems sounds like its in the tail pipe

car has been running bad for a while so i replaced all spark plugs and rubber boots and spring coils when i went to drive after that it seemed to have lost alot of power and noise was loud. upon inspecting the muffler system i found big holes in the flex pipe and the ctylac covertter had crumbling honey comb type stuff coming out of it so i replaced exhuast system from flex pipe back. now the truck backfires like crazy it sounds like its coming more twoards the rear of the tail pipe. any clues on what is going wrong i took intake upper intake manifold off and disconnected all coils turned truck over it appeared that all the fuel injectors are working properly. any clue please