This.
You say you are having a custom Spey rod made for a bucket list fishing trip to Alaska. I'm as sentimental as the next guy when it comes to things like guns, but I wouldn't let an old fly line potentially ruin my trip of a lifetime.
Whenever I come across a used reel, the first thing I do is chitcan the line, whether it's a fly reel or spinning/casting reel, unless the fly line is as good as new, but a 1960s or 1970s fly line ain't.
If the line has sentimental value, put it in some sort of display with an old reel and some flies.
