If you haven't found a solution I might know on the Tiagra in a few days, I've got a 39/24 crank arriving tomorrow and a braze on tiagra der and adapter arriving sometime this week. I would have thought the mtb fronts I have would sit low enough for the 39 but they don't. I plan to change the 39 to a 42 but that'll be a while till the bike is fully sorted.