I'm installing a vintage air unit right now. I also bought that square bulkhead
fitting but decided not to use it and go with the stock vertical unit, that came
with my unit. The problem using the bulkhead unit is that you will have to make all new line inside and not that it's that big of a deal but I like the hard lines that came with the unit.
I'll send a few tomorrow with the holes cut out.
Here is a pic of what i'm about to cut