Is it backfiring out the carb or the exhaust? What is the rule of thumb with backfires let see if I can remember... Out the carb is timing and out the exhaust is fuel ,something along those lines... Anyhow timing your car is very easy. Do you have a timing light? If not go grab a decent inductive timing light from Sears or your local parts store crawl under your 'stang and see if there are any marks on the harmonic balancer next to some numbers. That should be your setting go fire your engine up ,hook up your timing light loosen the distributor hold down bolt ,pull the vacuum line going into the side of your distributor ,plug it and aim your timing light down to the harmonic balancer. You should see a little pointer aiming towards the balancer align that mark with that pointer by turning the distributor clockwise to advance the timing or counter-clockwise to retard the timing once those two align you are timed! Well to factory specs at least... Should be somewhere around 8 degrees... Hope those directions were decent enough for you to understand I am not the best at giving directions but I try to help. Oh before I forget regarding your timing do you know if the cam has EVER been out of that motor could be some issues regarding that. Let me know if this helps any! 
Keep the wheels spinnin' and the beavers grinnin' i'm out.
Keep the wheels spinnin' and the beavers grinnin' i'm out.
