<VV> Missing/backfiring in a 110 pg

[Edelstein and Payne]

Folks,
     This weekend my son and I got around to installing the "trombone" dual exhausts that had been on the shelf in the shop for quite a while.  They replaced a stock but rusted out single exhaust on a 110 powerglide car that he uses to commute to high school.  Now when the car is cold, it tends to "hesitate" or "sputter", and if you press it, the engine will back fire a bit.  Once it's warm it runs nice, and even feels a bit stronger than before, and, of course there's the nice mellow exhuast sound that it now has (actually quieter than the rusted out stock muffler).
     I know that putting on a freer flowing exhaust can cause the carbs to lean out a bit, but that shouldn't cause backfiring, should it?  I didn't have time to check the timing when we got back last night from our "test run", but that's the next thing that I'll check.  What else might be causing it?

                       Thanks,
                       Travis Payne     65 Monza coupe
                       Raleigh, NC      65 500 coupe