I have noticed that they seem to all (at least the one I have and higher mileage ones in the junkyard) leak terribly after about 200k miles. I have an engine with 98k that is out of the car, so what could I do to keep the exhaust from getting covered in oil after I replace the old engine that is in the car now? I have heard reseal the oil pump, and something about a plate on the back of the engine. Anything else?
It's a Subaru, reseal everything.
Yeah Subaru gaskets suck at life, replace them asap once you get it.
I've owned a lot of old Fords and a few Subarus. I'm always shocked when one of my vehicles DOESN'T leak oil. 
The engine is out of the car? Yeah, do everything you can, even if it isn't leaking now. It's a lot easier to do now than it would be after you've bolted it into the car.
There is a plastic oil separator plate on the back of the block inside the bellhousing-y kind of area. It can crack and leak, and nobody fixes it since you need to pull the trans off the engine to get at it... so now's your chance.
Subaru has released two revisions of EJ oil separators (one steel, one aluminum) since that should take care of that leak.
http://allwheeldriveauto.com/subaru-seperator-plate-explained/
Between the oil separator and the oil pressure sensor, those were the two biggest leaks on my car. But then again, I was also paranoid-religious about the valve cover gaskets when I did those.
While it's out you might want to look at the valve adjustment; IIRC pre-96 is adjustable lash and post-96 is shims. It sucks to do it in the car.
The car I have is a 96, the engine is a 98. The engine that is in it has like 270k and smokes really bad (both from being worn out, and dumping oil all over the cat), but the new engine is still clean (came from a rolled legacy in the junkyard). I guess I will plan on doing all the seals I can get to, its kinda unpleasant to drive the way it is because it smells like burning oil constantly.
Keep replacing the gaskets. The H4s just love to leak out the valve covers. My cousin has become a pro at pulling the motors on Subarus, because it's way easier to just pull them out, redo all the seals and slap them back in every 50-60k than to work on them inside the car..
Yup, oil separator plate. This little video explains it:
https://www.youtube.com/watch?v=ByYR4asTnjQ
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