Don't know about the access difficulties specific to a '73; I've
done this in the car in a TR4A, which is essentially the same diff.
Dropping the diff is, in general, easy, and will make your life
easier. Don't hurt yourself.
While you're doing the pinion seal, check whether you need to
sleeve the pinion itself - if you can catch the groove
with your fingernail, put a speedi-sleeve on it, or the
new seal won't do you any good.
Strengthening the diff mounts from underneath is a whole other story.
Check out the new British Cars Forum: