Andy, I’ve used 34-28 on a Super Record RD, but agree with others that pushing bigger than a 28 wouldn’t make for a happy derailleur. If you want to cheat a little with a modern, but looks vintage compact double, and you’re feeling spendy, Boulder Bike has IRD 46-30 cranks for a pretty good price. It’s a really nice crank (put one on my/your Heron re-do).
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N = '96 Colnago C40, '04 Wilier Alpe D'Huez, '10 Colnago EPS, '85 Merckx Pro, '89 Merckx Century, '86 Tommasini Professional, '04 Teschner Aero FX Pro, '05 Alan Carbon Cross, '86 De Rosa Professional, '82 Colnago Super, '95 Gios Compact Pro, '95 Carrera Zeus, '84 Basso Gap, ‘89 Cinelli Supercorsa, ‘83 Bianchi Specialissima, ‘VO Randonneur, Ritchey Breakaway Steel, '84 Paletti Super Prestige, Heron Randonneur