I don't know if sorb0 has an early '99 or a '99.5. I'm not sure about the early '99 7.3, but the '99.5 is 15qts with filter change (with a stock oil pan). My '03 7.3L is 15qts with filter change and I'm at the high line. Maybe with the 16qts add not all of the oil had a chance to filter down to the pan? But with such a huge pan I don't think a 1qt difference is going to give you much of a level change on the dipstick anyway. I'd stick with the 15qts until your dipstick tells you that you are definitely below the line.
Just my .02. Hope this helps.