Been there ... done that. Capo #1, a Reynolds 531 frame originally built with 120mm rear dropout spacing, currently can easily take a 126mm "standard" 6-speed or a 128mm 7-speed. I encourage you to go for it. If you have a high-quality early 1980s bike, the frame is vastly superior to the components. This is even more true of most 1960s bikes.
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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