Long story short, yes it's easier/best to install the harness into the shell before it's installed and install it as a complete unit. It's also easier (but not required) to have the windshield out while you install it for ease of access to the forward dash frame screws.

The wire harness generally just lays along the bottom of the frame and is loosely held into place by hooks and bendable tabs, each "y" in the harness usually extends away from the main trunk of the harness toward whatever accessory they attach to. Pretty basic install. BTW, in 1972, the harnesses were not inique to 340 cars, all were basically the same.