You have imported a non-16:9 piece of media INTO a 16:9 project. If you HAD imported 16:9 media in the 16:9 project you would NOT be asking this question.

So, what's to be done?

I would start over and produce, what in essence you already have - an almost 4:3 squarish piece of graphics - and lay THAT onto a 16:9 plate in your favourite Graphics package. Then you would KNOW you are getting ALL of the material you can AT the screen ratio you have set your Project. I say this because all that detail needs to be seen - yes?

Pan/Crop does just that. It Pans and Crops. But like a "real" camera if it ain't in shot it WILL disappear of to one side. This way you get to have ALL your square-ish material front and centre. Yes you WILL have black bars at the edge, but at least you KNOW you have the option to then zoom- or crop-in.

Sorry, back to basics for you.