D1.5 deals with steels that meet the requirements of A709. If you check your CMTR you should find the A572 grade 50 meets the requirements of A709 grade 50. A36 normally will meet A709-36, and A588 will meet A709-50W. Just be aware that sometimes materials do not fully meet the requirements, so it's best to check the test reports to be sure.
That should help you to determine the proper preheat.