Well at this stage of the game,,i would double check my distributor, for proper installation,, also check the cap for tracking marks,,and i would be checking the Cam as well.
You had problems starting it to begin with,,so something was out of whack. hence the backfire.
And make sure you take a magnet to the inside of the oil pan to pick up any stray metal bit's.
Forewarned is Fore-armed.