You may also want to consider converting from 2x5 to 3x7. You could keep the friction shift, and possibly the derailleurs, as well.
When I converted the PKN-10 from 2x6 to 3x7, all I changed was the crankset (122mm BCD to 110+74mm BCD) and spindle.
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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