I started typing but this pretty much sums it up. http://www.homestead.com/1970mach1/standequip.html
The big thing were the beefier rear end, suspension, disk breaks, deluxe interior, and at least a 351w or 351c besides the other cosmetics. Not saying that you couldn't purhcase those options from the factory but it was just standard with the Mach 1 package. For example when my friend was 16 back in the days he had a 70 428cj fastback without the mach1 package.
Kevin