You are correct. Mtn cranks are set up to use a 50mm chainline, so a shorter BB spindle is required to pull it in to 45-47mm. I use 103mm with the 68mm BB shell, 44/32/22 mtn crankset. If you go the "Trekking crank", 48/36/26 you can get away with a 107-110mm BB. The 26 tooth chainring can usually be swapped with a 24. The Tiagra 4503 FD works well with Tiagra triple shifters.

This is the set up on my LHT. I have used it a 3 bikes so far, and it works really well.


I also built up my wife's new bike using the same combination.


If you are converting you might want to change out the rear Cassette to an 11-34. This will require a Mtn bike derailleur. Drive train retrofits are not cheap.