Transmission Downshift Problem

Hope someone can help.

Have a 1968 Dart 340 with 727, 8 3/4 with 323 posi.When trans downshifts under WOT into 1st it automatically shifts to 2nd and the motor boggs down. How can I get it to stay in 1st longer? Adjusting the carb rod will increase shift point but only under normal driving, not with pedal to the floor?


Thanks.