Re:
Samuel,
I'm a bit of a stickler for tradition and as I understand it TT Riders back in the day tended only had one front ring. A) to save weight, no front mech, etc and B) not many TT courses were particularly hilly so a wide range of gears were not required.
Many Top TT'ers, such as Alf Engers, would ride with just a large front ring of around 57 teeth and and a close ratio block at the rear, such as a 13 - 17 tooth through block. Once up to speed lower gears were just not needed.
Of course, as I previously mentioned, I would never state that this is the way you have to go. You need a bike that the are happy to ride and can enjoy. Nothing could be worse than having a bike that looks great but is a pig to ride, so go with whatever is best for you.
As for the Bottom Bracket (BB) cable guide. Most 70's bikes had a brazed on cable guide on top of the BB through with the cable runs to the rear mech. Something like this...
If this is not present then you will need a bolt on one such as this Campagnolo one which bolts to the down tube....
Or if you go the front mech route then one such as this Campagnolo one which bolts to the seat tube and has guides for both front and rear mechs....
Hope all of the above makes some sort of sense!