I'm not familiar with the 7/8 St. Croix, so I think someone like Dana would be better to suggest the line/weight specifics. However, my .02 is to consider one of the "mid-spey" types such as the Airflo Long Delta or Rio Mid-Spey. My rationale is that they have the best of both the long and the short bellies. They both load rods well with short lengths of line out (good for small to medium streams), while stripping will be required for longer casts there is minimal need for it on the mid-sized waters. Yet these lines will also allow you to shoot a great deal of line for the times when you may need to boom out a big cast.
I use an extreme long belly line for most of my fishing and do love how it fishes. However, I was given an Airflo Long Delta this spring and I fished it on the upper Squamish which is not too big -the line was perfect - I really liked how it loaded and cast, as well as how it handled on the water. I can definitely see myself using it on medium to small waters. I will also use it on the big waters of the Thompson when one of those nasty afternoon winds start to quarter upstream and the long-belly becomes difficult to use.
Tight lines - tyler.
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