I have a ' 01 R2000si with Ultegra 53/39 cranks and a 68x109.5 bottom bracket. I was wondering what bottom bracket I will need to change from Octalink to ISIS. The FSA bb dimensions seem to be either 108 or 113. I know FSA makes compact cranks that are Shimano compatible, but the price and availability for ISIS makes the switch attractive. (or am I getting into trouble?)

Use whatever BB spindle length FSA recommends with your crank choice. Just because it's shorter or longer doesn't mean your chain line will differ from your current set-up.