What's behind the B Pillar post base?

1970 Plymouth Valiant 4 door

I want to install new 3-point seat belts in the front of my son's car.
I think I'll be bolting the retractable part just below the base of the B pillar. (see photo)
Does anyone know what is behind there? I looked under the vehicle and the fuel lines don't seem to be in the way.
Also, how would I bolt into that part? I'd like to avoid welding, but can if needed.

Picture info: taken from rear seat angle. I'm using the power steering bottle as a pointer to show where the base would be bolted.

Thanks for reading
Danny