Seat and belt mounting should never ber attached directly to the tub of a prduction based car. It should always be attached to the cage structure.

The problem of the "third" mounting point discussed is it creates a place where the seat mount can bind an not allow the seat mount to move with the cage, especially the main hoop and the door bars. In the current SCCA cage rules you are allowed as many "touches" as you like so if you want you could put a foot on the seat mount at the right front corner but I'd only tack it to the mount and just let it rest on the floor pan.

Here's a couple of pics of how I do the seat mounts in production based road race and circle track cars. The pics don't include the belt mounting tabs, I don't add those on the seat mount tubes until I have the seat mounted and the driver in place to insure belt angle to avoid dumping.