No start on rebuilt 360LA

Having worked all day, trying every suggested fix. It is still not working. Continuing to backfire through the carb. We have changed carbs, distributors, wiring, troubleshot the msd box, checked all 8 spark wires for spark, coil wire has spark, moved plug wire positions, rotated 180 then back 180 multiple times. Set initial timing, adjusted valves, you name it, we spent 8 hours doing it. Thanks for all your help fellas, but I am out of ideas.