I've seen cars without seatbelts and there were dimples. Had to drill holes and putting a backing plate/nut.
My 62 Valiant had seatbelts in the front with those loops, backing plate/nuts behind them, but they were directly behind the seats on the floor.
Right where rear passengers would put their feet.
Seatbelts were also clearly not factory.
I think you're going to have to wing it.