Under the largest portion of the longitudinal member of the tube frame. Be sure that you are well under the tube frame and not stressing the rocker panel or floor board.
If I am going to raise all four wheels, I put the jack in the middle of the car and then place the jack stands about four inches in from where the fender wells meet the rocker panels. Jack one side, put the stands in, then do the other.
It's not a bad idea to have the jack in the "up" position under the portion of the car where you are working...Just in case the car slips.
It helps if your floor jack and stands have rubber pads that prevent scratching/marring the surface and slippage when taking up the weight of the car.
TMc