Wheel dishing is easy. Start with a straight frame and install the wheel forward and then backward. If the rim's/tire's lateral position does not change relative to the frame, your dish is correct.
I ran a low-end 1962 Bianchi 10-speed (120mm rear axle overlock dimension) as a 3-speed coaster for awhile and don't recall having any issues with rear triangle width.
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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