440 Won't start syndrome

You need to pull the valve covers and make sure that all the valves are closed when they are supposed to be. could have jumped a tooth if the timing chain is loose. Any idea how many miles are on the engine? it could be a clue to how worn things might be. I have also seen the outer rings of stock balancers slip on the rubber. Maybe what you think is 5 degrees could be way off. Keep this in mind, it will only backfire thru the carb if an intake valve is open when spark is introduced. Like wise if it backfired through the exhaust, this would mean that it fired while an exhaust valve was open.