Make sure pcv valve and all other hoses that are vacume are still hooked up as well as check intake gaskets and valve cover gaskets. Sometimes they can get blown off when it backfired. Do you have a compression guage? You should check the compression yourself to verify it is low. Check the timing also and make sure all spark plags are getting spark. Run it in the dark to see if any spark plug wires are jumping spark. Any gas in the engine oil? You can smell it on the dipstick. Keep posted.