If you determine the tubes are metric, then it's definitely 753. No way would Jim have used metric tubes for any other reason.
One clue, not conclusive by itself but indicative, is if the BB shell is a Cinelli investment cast that says "FRANCE" in the flat spot on the bottom. The shells weren't made in France, that's just telling you it's a French shell as in threading, and metric sized tube sockets.




26.8 seatpost is also indicative of 753. Normal French frames with metric tubing (531 or Vitus) take 26.4 or 26.6 posts. A 26.8 post in a 28.0 tube is possible with 531 but only just barely, and uncommon. Like putting a 27.4 post in a 28.6 tube — you do see it in 531 sometimes but more often you see 27.2, with 27.4 more likely to fit in 753.

Honestly though I'd hope it's not 753 if it were mine, due to how whippy that would be in such a large frame. So my money is on 26.8 being the seatpost due to a heavier 28.6 seat tube, chosen for adequate stiffness for such a large rider. Did you say it was a 66 cm? I don't think 753 seat tubes even came that long.