Make sure you know which box you have before simply replacing it. If you have a round top then you have the 600 series standard box. But I was under the impression that Calais got the 800 series 4 bolt top box standard.
If you have the 800 series with the 4 bolts on the cover and adjuster in the middle, you're in a great starting place. Still, the stops may be different but a 2.5 lock to lock box will bolt right in.
Reason I say this, is you can't mix and match the pitman arms that connect to the box. You have to get a pitman arm to fit whichever series style box you install. If you're simply replacing like for like, you're ok as is. But if you have one and change to the other, you'll need to get a matching pitman arm to fit the new box.
New pitman arms for the 4 bolt box can be had for about 79 bucks shipped if you need one. If you already have a 4-bolt top box, you won't need to get one.
http://rover.ebay.com/rover/1/711-5...0001&campid=5335822911&icep_item=290878717984
Plus, if you have an old steering pump mated with the newer "metric" box in your 83, you have to have a different hose or adapter or something. The old stuff uses inverted flare while the newer boxes use O-ring seals.
But first, diagnose the issue as to WHY and WHERE you're actually getting the slop. It may be one place, it could be many places.