Gary & Frank are good first choices. They both have been known to cause the backfire issue you are speaking about. After cleaning out the distributor, double check your timing as well. If all else fails, pull the head and send out for a rebuild. We have seen bad/leaky valves in hard run trucks do this too.
This is ONLY to be used to report flooding, spam, advertising and problematic (harassing, abusive or crude) posts.