Seat installation
The Rules
The rules are pretty much the same across sanctioning bodies (ARA/CARS/NASA/SCCA), an FIA homologated seat must be used. CARS is the only sanctioning body currently enforcing expiration dates on seats (5 years recommended but they allow up to 10 years from date of manufacture).
Seat sliders are pretty much banned across the board even though FIA allows them (if they are FIA homologated which pretty much none of the available sliders on the market are).
ARA rules are below but the FIA Article 253 rules have much more details about installation instructions.
Choosing a seat
You can base your choice by doing careful measurements (most manufacturers will give you exact dimensions inside and outside). The seat must be able to fit in the car but most importantly you must fit comfortably in the seat. You won't really know how the seat fits you until you really try it. Some shops carry some seats that you can try out, you can also go to a local event and ask competitors if you can try out their seat if they have a model you are considering, most of them will let you try them out and give you feedback on their experience with the seat.
Consider a seat with a Halo as the Hans device is only a Frontal Head Restraint (FHR) and the keyword here is 'Frontal'. This means that it does not protect you from a lateral impact (like hitting something sideways). Only a Halo in the seat will stop your head from moving laterally and hitting the side of the car.
The position of the driver is important as shown by the FIA Article 253 diagram below:
Installation
The seat must be secured to the car by at least 4 mounting points. It can be bolted directly to the chassis (see below left) or another popular option is to use a seat mount that re-uses the factory mounting points (picture below right). The depicted Planted seat brackets are a low cost popular option: https://www.plantedtechnology.com/products/seat-brackets.html
An alternative is to have new crossmembers to support the seats. They must be secured to the chassis using end plates. The pictures below show example of seat support crossmembers. Note that the bar can be a square or round tube but it has to be secured to the chassis and cannot be welded to the cage by FIA standards (it should also be straight and not be bent).
The FIA specifications for new seat mount bars can be found in Appendix J Article 253 with diagram 253-65B detailing the specs of the different components:
