If it makes any difference to you, the Gran Sport can very likely handle 6 cogs, up to 28 teeth (or more). No good reason to back to 5 speed, unless you want the 6-speed freewheel on another bike.
Are you just trying to put the less-desired components on the too-big MkII before you sell it? Not a bad strategy if you want to build up the smaller one as your main bike.
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● 1971 Grandis SL ● 1972 Lambert Grand Prix frankenbike ● 1972 Raleigh Super Course fixie ● 1973 Nishiki Semi-Pro ● 1979 Motobecane Grand Jubile ●1980 Apollo "Legnano" ● 1984 Peugeot Vagabond ● 1985 Shogun Prairie Breaker ● 1986 Merckx Super Corsa ● 1987 Schwinn Tempo ● 1988 Schwinn Voyageur ● 1989 Bottechia Team ADR replica ● 1990 Cannondale ST600 ● 1993 Technium RT600 ● 1996 Kona Lava Dome ●