\... I also do not know if a single, new 8sp chain would be long enough.
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New chains are sold longer than most riders need, so it's a matter of cutting extra off. The chain has to be long enough to loop the big/big + 1" to spare, and short enough that when on the small/small the RD can take up the slack. On most bikes there's an inch or two between these, and you have some latitude. I prefer to us the longest chain possible, others the shortest, and Goldilocks uses something in between.