It wouldn't matter so much since you are using a single throttle, so your plenium is in fact a big balance tube, I would mount it as close to the throttle body itself, so the pulsing effect that hapens before echt runner doesn't effect the map reading.
You want the map signal to be the average signal of 6 cilinders, and you want a smooth signal, not pulsating like you would have if you'd mount it on a single runner. so choose a place where the signal is not affected by the pulsing runners.
Only reason i say in the balance tube is after datalogging on a car with a jun plenum there is a small difference in pressure vs throttle opening speed (Balance pipe to map sensor just after throttle body).
It also made small throttle openings nicer (drive wise).
I'm not talking massive difference and i may be own my own on this one but i'll stick with the balance pipe on mine.