"Chromoly" is a combination of the names for the two major material types used in making the material that goes by that name. Easy to look up all the particulars online if you are interested. I use ARP header bolts in everything I own with the exception of the stock factory cars. I don't remember the size I used on the 98 but yes both engines got ARP hardware including the header bolts.
I used stage 8 locking stud fasteners on mine to be sure they are not working back out. There are two additional pieces that go over the nut after it is installed. One is a clip that goes around the nut and holds it against the header pipe and then there is a clip that slides over to lock that other one in place.