For the spindle I'd find the part numbers for both the BB and the cranks you purchased and try to find them on Velobase. You may be able to see what the standard equipment is for them and compare. You'll need to measure the spindle with calipers just to know what you're starting with. I bet if you found the cranks on Velobase it would give you some options for BB lengths that came equipped with those arms. Looking at that, knowing what you'd have I'd find the next longest recommended length or more, depending on how off you think you are. I enjoy the educated guesswork =)