Interesting. I ran my first Bianchi road bike, a low-end 1962 Corsa, as a 3-speed coaster for awhile, as a campus commuter, when I lived within a couple of miles of UCLA and needed something theft-resistant. I used one of the stock Huret downtube shift levers, which I think I would prefer over your admittedly very clever seatpost mount. I also like the old-fashioned stick shift quadrant shifters Sturmey-Archer made for top tube mounting. 54-72-96 sounds like good gearing for flat country; I used something closer to typical English 3-speed 50-66-88 to handle hills around here. That black PX-10 frame looks fabulous.
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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