Question about middle lap belt.

[spookyu]

I have a 69 C10 longbed that I'd like to replace the seat belts in so I'd feel comfortable taking the family around in it. I'm just going to buy regular 3 point seat belts new but I'd like to install a center lap belt. I took a quick look under the seat and didn't see any kind of attachment point for a lap belt in the center like if this was an option that the original owner opted out of. Was hoping someone here might have already tackled this simple problem and might have some advice for me?