Are you running an open hole spacer? On a dual plane manifold? If so why? Yes, an open hole spacer can help a way too small carburetor make more power at upper RPM, but it will play hell with tuning everywhere else. You're no longer trying to tune a 600 CFM carb on a 273, but more like a 1200 CFM carb on a 273. The open hole spacer literally cuts the vacuum signal at the carb in half. Ditch the spacer. the fact that adjusting the idle mix screws does in fact have an effect means that the idle circuit is in use at idle and is functioning correctly. The fact that you have to screw them in about twice as far as is typical also indicates that he vacuum signal is weak. Yes, you can make it idle, but because the signal is weak the primary circuit isn't working well at cruise. So you end up with a dead zone. The idle circuit is cut way back, but there isn't enough signal (airflow) in either primary barrel to mix the fuel correctly. You're literally between circuits.
When you say "backfiring" on trailing throttle and cruise, you mean popping from the exhaust, right?