If I wanted a Herbie that can handle 400 gph, I'd probably drill for 1" bulkheads but use bushings to bring the actual plumbing down to 3/4".
Sorry, in what context? If you drill the bottom or side of the tank, you would typically then build an "internal" box around the holes and put the standpipes in it. In that arrangement, an internal box is definitely part of the design. If you're talking about a hang-on overflow, then the internal box is what sets the water level at a point higher than the entrance to the siphon tubes, so it needs to be there (to keep them from sucking air and breaking siphon)
Oh yes, I see how 1.5 is overkill for the siphon drain. Would you recommend a 1inch drain for the emergency as well?
Since this is my first time, do you happen to know of a really detailed guide on how to do plumbing? ie, which connector needs a slip and which needs a screw fitting, which ones have to be glued together, details like that? the closest I could find was a picture from beananimal.
The reason I asked if I needed an internal overflow box was because the herbie guide I saw said to put the siphon drain at least 6 inches below the emergency drain and the only box I found had a height of 5 inches from bulk reef supply. I figured if I just drilled 2 holes and attached pvc directly, I could achieve that.