Ok.. if your diff came with a washer or you had a washer for the old diff... You should be able to measure the two diffs instead of the clearance to the case. (assuming you didn't swap non-hd to hd). If you are going to HD and the bearing and washer are there, use them and smile when nothing bad happens. (Seen a ton of people do it this way).
On the diff being able to flop.. it can't. This extension we see on the top of the diff. It rides in the bearing in the diff cover. This diff has the bearing and selected washer on it in the picture.
