My first thought was improperly installed distributor. Most of the guys have pointed out the major reasons for backfiring: I would check the distributor and wiring again for proper firing first. You didn't mention when this started and if you had done any work on it before this happend.

Distributor installed wrong (see manual for proper installation) change points too.

Improper plug wiring (18436572)

Stuck valve (remove the valve covers and watch the action with the rotor out as stated) and compression -leak test

bad cam or timing chain (procedure same as stuck valve)

bad vacuum leak?

I wrote this out for input and corrections so I could file this in my archives.