Wow! That a huge plant! Nice buy. I would gently loosen the roots with my fingers and get the ones traveling up to go down. You can cut it back by a third and use the fresh basil now.

Nice plant. The roots aren't too overgrown.

I'd cut off half the leaves for cooking and plant it as it is.

The narrow window box looks a bit small for such a large healthy plant.
As suggested, i would take off some of the top growth stems and and use in the kitchen.
You could also take some of the longer stems, with leaves and root in some clean water and make more plants to put in your window box.
I would get a heavy larger pot to re-pot and keep pinched back for all summer picking.
My local market sells big healthy ones like that. A few years ago i kept one going all the following winter in a south facing window by keeping it pinched back....it grew very full like a small shrub.
If you want to use your window box, just add more soil and keep it moist for the first week while it adjusts, and give it a haircut. When you cut, take the top stems off right down to the next set of leaves below...can also take a few lower individual leaves...