engine running rough and backfiring with PO300 code flashing. on 1997 GMC Suburban 1500

have replaced intake gaskets(upper and lower)plus all other gaskets up top.NEW (plugs,wires,cap and rotor replaced mo ago)brand new ign mod,,coil,maf sensor, crank sensor,EGR,catalytic convertor,all O2 sensors,injectors were removed and cleaned,...replaced all of this stuff and still engine backfires,have adjusted timing traced every wire,vaccuum hose...checked and double checked each and every electrical connection, and then some...still backfires. have bent accelerator cable bracket to increase idle a tiny bit... nothing is working... still back fires,, i am at the end of the tool box.. dont know what else to do..talked to mechanics friends etc.. they are baffled too..someone please give me a new direction to look....any help would be appreciated

Agree with answer 1. If that is not the problem then replace the distributor assembly with a NEW one, not a reman!! Be sure to get it back exactly as it came out! Dont get it even a tooth off and expect to just set the timing to compensate! Follow procedure for ignition timing listed under the hood!
2 more answers
make sure all the plug wires on the correct towers. make sure fire order id correct.
I had one the plastic tees in the heater hoses break inside of the hose where it could not be seen. whenever I tried to the expressway under acceleration the engine would stumble and shake. the code read multiple cylinder misfire because coolant was being sprayed on the right cylinder plug wires.