I'll just toss this out for thought, I suggest that you consult a professional structural engineer for this project. Reason I say this is none of us know what the loading is or could be and at what point in the span. For any of us to offer a recommendation could subject us to a potential law suit should something unforeseen happen. Very much like the safety switch issue. You may want to consider something like a moveable gantry crane or an engine hoist with a rating on it. I use and engine hoist with wheels in my shop. That said good luck with your project.
