I've noticed nobody has considered the weight and length of the
fly-rod as a factor for determining fly-line size. At least that's what
they used to do before the days of writing line-weights on the rod.

Just about every manufacture determines a "suggested" line weight for each
fly- rod that they think would be best suited to make the cast. However
depending on your casting style, it could easily vary between anglers.

What is also given is the actual weight and length of the fly-rod.
I was going to ask Brian for the weight and length of his 6wt and
perhaps I could help him with line selection. But then I read that the
rod cast like a dream with a 5WF line, so I just left it as is.

From reading all the responses now, I think it's best for the angler (Brian)
to experiment with the different lines to determine what will fit his needs.
But it's a good that everyone wants to help although we sometimes disagree.
-tom