Hackler,
I called St. Croix. They do most of their testing with Scientific Angler Mastery Series. If you want to overline your rod they suggest only going up one size, to 6 wt. Some lines are 1/2 extra and that is also ok.
Personally, I would use the Rio Grand 5 wt since it is designed a half weight heavier, or my favorite the Rio Gold fly line.
Larry ---sagefisher---