Most DS loops mount to the rear tranny mount or near there depending on the car. The loop must be close to the front universal because if it were to break there the DS could go down and catapult the car,
Should the rear one give out it will flop around and destroy the underneath but not catapult the car.
Two thresholds for DS loop
One if you are running Drag radials or slicks, you must have a DS loop regardless of speed.
There is a speed/ ET standard as well, bit IDK what it is.....most people met the first criteria
I've raced IHRA and NHRA, street classes, so thsats where my knowledge comes from....your situation may be different.
If you have ever seen a DS universal break you would probably want a DS loop in all your cars...
I always get a kick out of the outdated and unclear NHRA rulebook.
For all full-bodied and open-bodied cars running between 7.50
(*4.50) and 13.99 (*8.59) in place of a crossmember located
behind but within 6 inches of the center of the front universal
joint: A front driveshaft loop is required on all cars, except
vehicles running 11.49 (*7.35) seconds or slower equipped with
I've seen that interpreted as "drag radials are street tires" and no DS loop required. Which is a bit ironic, as no tire is harder on driveline parts than a drag radial! Slicks are a forgiving marshmallow in comparison.