There are some other possibilities for the location of your leak. Some AFPRs are sensitive to fuel additives, especially injector cleaner or octane boost. Fuel can leak from the vacuum port into your intake. This would also cause the backfiring. This happened to me.
Pull the vacuum line off of the AFPR and then have a friend turn the key on and off a few times to pressurize the fuel lines. See if fuel comes out of the vacuum port.
Another place to look is the seal at the top of the injector where it meets up with the fuel rail. This is less likely since you would notice fuel pooling on your intake unless it's happening under the plenum. Does your engine compartment smell like gasoline?