Spotting to the floor pan is much stronger but may not pass tech. The floor pan is just there to keep water out.
Try to hit the sills and floor corners where multi pieces come together. Running a tube, sitting on the sill connecting the front and rear cage down tubes is the best way. Just on the driver side is fine.
When you fit the seat, allow the seat mount tube to lay on the floor and connect to the cage legs and or door bars or prior mentioned sill tube..
Ideally you want the driver box to push out of the floor taking the floor and cage assembly with it.
Modern cars have some strong points built in. Three layer areas, extra thic tin here and there. Play tappy tap with a small hammer to find them. weld to them.

As far as tubes joint and such, small angles are much stronger than 90* angles. triangiulate where you can and run straight tubes as opposed to bent tubes.
IE pass door bar can be straight and much stiffer than bent door bars. sameon the driver side, one straight angled tube is way stiffer than bent door bars, if you have room.
https://www.youtube.com/watch?v=1uJ9XCFZBvA This dash bar is angled across the car to help with stiffness.