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

The factory seat belt mount should be under the carpet right about where the end of the bottle is, no need to drill any holes in the B pillar.

Thats right, just roll up the carpet a bit. you can see the weldnut from below.
Since seat belts were mandatory in 68? already,why is there not a hole in the carpet, already? New carpet?

There are seat belts in the car already. Two per side. One lap and one shoulder belt.
The shoulder strap is bolted just above the rear doors while the lap belts are bolted further up on the floor. My son is at symphony practice now, but when he gets home, i'll verify the mounting location of the front lap belt.
I do believe it's new carpet. It was originally a green car with tan interior.

I found where the current seat belt is bolted. It's closer to the front of the vehicle. Between the center of the B pillar and the rear of the front door jamb. (see pic).
The retractable (3 point) unit I have is larger and will not fit between the seat and B pillar; which is why I was going to install it where it was shown in the original picture.
Depending on how thick the B pillar sheet metal is, I might be able to squeeze it between the seat side and the B pillar (up higher). My son is the only one that drives the car, and the seat never gets adjusted.
thanks

Picture info: taken from the open passenger front door.