No, both SAE Gross and SAE Net horsepower are measured at the flywheel. The difference is that Net horsepower is measured with the engine configured and equipped more in line with an actual vehicle installation. Read more about it here.
Also, larger tires (14" from 13") and lower ratio axles (like 2.76:1 from 2.93:1) were used
Yes, and the cars grew heavier over the years. Bigger front bumpers and side impact beams in '73, bigger rear bumpers in '74, reinforced windshield headers in '75, etc.