1967 Dart GT Wiring Questions

Later models had a ground wire on the headlight switch and that's the type switch aftermarket provides today. So if you buy a replacement switch it will have a extra male spade terminal at the back of it where you could add a ground wire. Majority of your OEM build used only the mounting hardware for chassis ground. Very little of the instrument panel will function while it dangles from the harness connectors which is a good reason to add a ground wire to it.
Your OEM fuel sender ground was a hard sheet metal like jumper clipped onto the fuel tubes jumping over that short piece of rubber hose. You should have seen this in changing the sender. If/when its missing, owners would use a length of wire and 2 small gear screw clamps to create a jumper.
I don't recall was the tag light wire black or brown but that and the blue wire to fuel sender were the only wires through the trunk floor.