"One of the lines I ordered arrived this AM. It's the 34ft 510 gr Scandi line.
But, as it says, it's 34 ft long. What goes next on the reel?"
For clarity, is this a whole line INCLUDING a shooting line behind the head. Or just a 34 foot long 'head?' If the first all you'll need is backing. 125 to 150 yards of 30# dacron, or what ever you like. EXCEPT Gel Spun
. Stay the heck away from that stuff on a fly rod. Stuff cuts like a knife.
If the second you will have to get a shooting line. Lots of brand choices out there, personally I use RIO's. But some folks prefer Blonds, some Redheads, etc.
That said, all the one's I've seen are 100 foot long and usually come 'pre-looped' to connect the 'head' to the shooting line. Only choice beyond that is 'how fat' do you want same? Personal opin here only but with 5wt and down I use the .024, 6 and up the .030. Suspect, other than one's 'fatter' than the other the real deal may be the core is stronger on the .03.
For cold weather work the extra width really helps!