Depending on weather you are trying to be safe or pass tech depends on what I would suggest doing.
If you just want to pass tech, then do witch ever as it will get you into the track.
If you want to be safe then I would highly suggest that you mount it at spot #2 and then make a second plate welded to the sill were the seatbelt mount is. On my car I then drilled a few holes to roset the plate in for additional strenght. The sill(side) of the car is the strongest point in the car. Spot #1 is just behind a body seam the runs the entire lenght of the car and probably not the better choice for mounting. However either is better then having no cage at all.
If you, or anybody is building a cage. I would suggest a NHRA rule book. They are very expensive and hard to find...... lol
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