Things you did/upgraded that surprised you how helpful they were

Started off with a 67 Satellite b-body. Best ET was 11.85 in the quarter. Switched the engine, transmission, torque converter, rear end center section, front tires, and all four wheels to a 75 Dart Sport. Only changes on the Dart was smaller rear tires, new shorter driveshaft, and electric cooling fans to clear the water pump. With the same carb, same ignition box, same spark plug wires, and same tuneup the Dart Sport ran a best ET of 11.46 in the quarter.
I would say this is a clear example of weight difference allowing the Dart to run quicker. Not sure if the aerodynamic properties come into play much at this level, but it improved more than the two tenths or so I expected.