At a quick look of the Rio Website, it looks like they changed the design on the 6/7 line, Mine I could swear was in the 400 range--410 I believe. I purchased last year, now the same label 6/7 looks to be 500!
The 500 is going to be way too heavy I'd suspect. I wouldn't put that on my 6/7.
If I were in your position before buying ANY line, I would contact the folks at Wild Water and see what the grain window is on that rod. I didn't see grain windows listed on their website. You really should establish this before getting yourself a line otherwise it's going to be trial and error. The other problem is that sometimes a rod labeled 5/6 might not be truly a 5/6 so trying to translate a grain window based on what stamp the rod has on it is not always 100%. Ask Wabi about his experiences with his switch. He wound up with a pretty hefty rod that was mislabled if memory servers me right. If you don't get the results you're looking for there, Steve Godshall is the guy to talk with. He makes custom lines and you're likely paying similar price to new line but the advantage is he knows what lines work with what rods. Lots of folks spend a lot of time and effort trying to match lines with their two hander and you'll often see guys selling them. I have not dealt with him personally, but fredaevans has. I've read nothing but great things about Steve here and on other forums.
Mike, I'd be happy to let you try the 6/7 line if you'd like to before going out and dropping the funds on it.I'm loaded up with eggs and nymphs. Only problem is my work schedule is M-F, but I could use a day off and a day to fish. I could look into taking a day and heading up to either Elk or out to Ohio--which I know NOTHING about. I'd just need to know which day you're off and are able to go.
Once you establish that grain window, you can go line shopping. Here's another option for you:
Hook & Hackle " Switch 101's " Fly Lines-Hook & Hackle
If it's anything like the rest of H&H's lines, they're going to be Cortland made. I bought one to do practice casting with and it's not a bad line.
Hoping one of the other members with much more experience with 2 handed rods and lines can chime in here too. I'm still learning but from what I've gleaned over the past year, I think I'm close to if not giving you good info.