Short or long cage refers to the spacing between your rear derailleur pulleys. A long cage derailleur is generally used with wider-range gears and requires a longer chain. A short-cage derailleur is always used with narrower-range gears and can use a shorter chain.
I like SRAM chains and hear good things about Whipperman -- just make sure you buy an 8 (or fewer), 9, or 10-speed version, according to the number of cogs on your freehub/freewheel.
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"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069