I know of 3 different vacuum hoses that the 3,9 has, but haven’t a clue of their inside diameter. So letting the Forum know the application the hose is for could help.
If I correctly remember, the cruise control vac hose which I replaced was 7mm ID.
The fuel pressure valve pipe is only around 16cm long and is moulded to for a bend in the centre so no kinks, and its obsolete of course like the dizzy pipe, this pipe is thin around 5 or 6mm, at the end there’s a rubber elbow plus a straight connector to enabling the pipe to attach to two nipples that require a larger diameter.
The part numbers as far as I can make out were ETC7313 & ERR4676