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Not sure what you mean - 700c wheels are slightly smaller than 27" wheels so if anything the 700cs that're on there now would make the clearances larger. Anyway the diff is too small to call - the wheels on the bottom pic are 27" and the current ones (on top) are 700c.
Seat tube is pretty standard from what I can tell but as you'd expect it's all French sizing. Have put a pic up below.
'Nice tight clearances' - is it meant for 27" wheels then?
Does it have the signature Salmon seat tube recess?