What is this box?

It's the actual seatbelt interlock box.
74 and newer. I think they went away at some point but I have a 77 b body with it also. And my 74 dart.

You can leave it and just leave the seat plugs under the seat unplugged. It'll kill the interlock requirement.

If you must remove it, there are 2 wires under the hood, running to another little interlock piece, that need tied together. I want to say yellow, and yellow/brown tracer.

If you Google mopar seatbelt interlock removal there is a article on BBodies on it.