Yes they were bored as a pair but with some care you can mix and match, it has been done successfully many times when needed.
Many 850s got converted to remote way back when, also I've done it when the diff housing was badly cracked in a prang, another when the housing was missing.
Bolt it up with the 2 paper gaskets in, get an inside micrometer or an external one and some engineers' telescopic gauges.
Measure each side at the 12-6 o'clock and 9-3 o'clock positions. Write down the result.
Measure a diff bearing, and compare to the figures above.
This will show how far it is out.
Ideally, you want the 2 housing bores the same as the bearing or .001" smaller. NOT bigger.
If you need to move the diff housing vertically, you can leave out the 2 dowels. They are there to locate the housing during machining, once the housing is bolted up they are not essential to its function.
If it is too large horizontally, the diff housing can be set up in a mill using the figures, and machined to suit.
Have fun.. it's not hard. If you can't get it correct, try another diff housing.
