Unless you insist on indexed shifting, try converting to a 7-speed freewheel. With some 6-speed hubs, the operation is as trivial as unscrewing the 6-speed freewheel, removing the pie plate spoke protector, screwing on the 7-speed freewheel, and touching up the rear derailleur stops. With others, such as my circa 1980 Campag. Gran Sport hub, you would also have to add a spacer or two to the right side of the axle and redish the wheel a bit.
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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