Edelbrock 1405 ( 600 cfm) on a 273

Get a $150 wideband meter. you can instantly tell wha the carb is (not) doing and will direct you to a solution. On a Carter its a completely different principle of operation than a Holley when it comes to metering but you should nail your issue. The 600 as all carters/Rochesters were velocity sensing so they were very forgiving on mismatched CFM's. Holley DP's were opposite, open those toilet bowls mechanically and its go time whether the motor was ready or not.