On ours, it's the outside edge of the bracket usually. Unfortunately too, whether the inconsistency is with the radiators or the Broncos (not like
that would ever happen!

) some get away with just a quick trim, while others have to trim off quite a bit of the mounts.
It's often both the bottom and the top "clamp" parts, but is pretty straightforward.
Here's a pic of one that was taken just in the last week or so.
He's got a replacement aftermarket core support and the Ron Davis radiator and only had to trim one edge of each mount slightly. One of our own Broncos with all of it's original body parts had to be trimmed quite a bit.
You can see how close the shroud is to the inside of the mount too. He's planning to trim that a tiny bit for more clearance. Even though it's stainless against aluminum, it's nice to have some gappage between any dissimilar metals.
Just so you can say you did...
Paul