What is the diameter of the fresh air cowl vent opening?

To everyone out there that is smarter than me (which is pretty much everyone) do you think I'd be going backwards if I used the plastic Mustang repair kit that is 5 1/2" in diameter? The insert has a plastic, top hat rim that could be trimmed if need be.

I'm still pondering how to get up inside there, and stop the rust. I got that funny feeling it's going to draw a little blood when I reach in from underneath the dash. :BangHead: