[Tom Goldman]

This is required on most Top Sportsman cars .
I would think if you are racing a 25.1 chassis car in SG you may be requested /or required to add the enclosure.
There are already requirements in place for cars where the driveshaft passes the driver and is not protected by the stock floor, altho it is vaguely written.
I've been putting a 1/8" tunnel enclosure on cars I build for years after seeing a racer have his arm severed by a driveshaft failure in the water box.
If you have a 9" rear ,Bickel racecars has a very nice bolt on rear enclosure ,and also tunnel tube kits.
It may not be required yet,but it should be.