I personally have never heard of 631 Araya. Reynolds 631 is a manganese molybdenum steel, and the numbers represent the percents of the major alloying material, so this number could be the same type of thing. Araya wouldn't have hung 600EX parts on a hi-ten steel frame, so it is more than likely good butted chrome moly. Take a picture of the tubing sticker when you get it, I'd like to see that. The parts group is cool, and will shift over seven speeds very well (I have tried it.)

Really, I'd be suprised if the derailleur didn't have the throw to cover eight speeds. Shimano tuned the arc of the derailleur parallograms to match the spacing of the cogset (or maybe the other way around...) when they went to indexing, but if you are sticking with friction shifters this is not an issue.