66 Barracuda A/C vent to dash dimension

So the placement under the dash is dependent on the AC unit
It does and to some extent on the factory location of the AC/heater unit bolt mounting holes on the firewall . There is some wiggle room (not much) at the vent into the opening of the AC output location. My vent slipped right into place at the AC and also to the vent mounting area. Maybe cause the heater/AC unit fitted right into place on the firewall and vent right to the dash. Must have got really lucky as I cut out the original non-AC firewall and welded the AC firewall in after lots of measurements