Stock or modified belts? 3- point (stock lightweight belts with a stock 2" lightweight diagonal shoulder harness) arguably makes less difference safety-wise than new aircraft quality 3" wide vs stock 50-yr-old 2" wide seat belts.
If you add 3" wide heavy duty belts, the stock diagonal shoulder harness (either early or late) cannot fit so must also be replaced. Removing the now-useless stock 2 or 3 retractors will be your only weight savings.
Crow, Simpson or a few other systems with a SEMA approval tag have been successfully used in Panteras for street or autocross competition up to LeMans. Beware of cheap far-east counterfeits!