Is exhaust back pressure the culprit here?

My mistake, Aaron just corrected me. The 3" header collectors were already reduced down to 2.5" exhaust pipes and mufflers. This final trip took 2.25" after the mufflers out the back.
Brief history...
New engine was broken-in on dyno stand, Holley 750 was tuned, and timing was set. Engine was installed and timing was off, apparently due to differences in MSD dyno ignition and the Dart's stock Mopar ignition. Car was running good, but it died on short trip from neighborhood and refused to restart...until three days later when I retried it out of curiosity. Would start every time afterwards, but now I had no idea what the problem was, and certainly no confidence it wouldn't do it again.
Decided to try Trailbeast's HEI conversion and the timing advance limiter plate by 4secondsflat. Engine re-timed where it liked 16 initial and distributor limited to 18, for total of 34 advanced.
Put over 200 miles on car with this configuration in all kinds of traffic conditions and with multiple stops and restarts. Drove it 30 miles to muffler shop and the above described change in idle was noticed. Car runs good once warmed up and going. That's why the question about whether newly introduced back pressure would require slight adjustment in timing or carb.