1993 Chevy Blazer 4.3

Truck started off not starting every time, running rough, and dying. Changed distributor cap and rotor, plugs, wires, throttle position sensor, ignition coil. Still same problem. Then the fuel pump ran continously while truck was running. Changed fuel pump relay, still same problem but didnt pay too much attention to it. Changed the ignition module on distributor, after that havent got it to start once. Ive done it over and over again. Get it top dead center, make sure rotor tab is pointing to #1, put on plug wires in proper order (165432) still will not start and sometimes backfires, so I flipped distributor 180 degrees, still same problem. Left it alone for a few days and now the battery is drained. Its a brand new battery. Its getting spark, air, and fuel. I dont know a whole lot about cars and im out of ideas, from the websites ive been looking at it could be a crankshaft sensor, computer, camshaft sensor. The list goes on and on. Any help would be appreciated.