Exhaust backfiring

[Mv7fd]

I took my truck to a shop to get the carb rebuilt and timing redone they also did some egr work and vacuum line work and they call me and say due to the exhaust it'll still back fire some. Is this possible it's a stock 350 with long tube headers that run into flow master 40 series and come out stock location the exhaust looks stock besides the pipe diameter is bigger and it's a true dual exhaust with headers