If you are naturally aspirated this is simple Do you have a firewall and is your trans completely covered by tin? If you do Then it is a -5 suit If not it is a 15. It doesnt matter if it is an aftermarket case or not, the above rules apply
If you are running a head restraint then you still need some kind of fire protection for your neck. Either a sock under your helmet A skirt on your helmet Or a neck collar along with your head restraint.