So, I'm in a grump mood. I changed out the engine because lots of people told me "yea, that sounds like rod knock, it's doomed".
I started taking the old engine apart today. I pulled the oil pan and checked the rods for play. None of the rods had any play. The crank doesn't have any play. But, one of the timing belt idlers I put on (an almost brand new, nice one) had lots of play. When moved by hand, it made the "rod knock" sound.
I still have no explanation as to why it didn't build up oil pressure immediately, though. It would start and the light would be on for 5 seconds as it made bad sounds. Then the light would go off and it sounded better.. Probably bad oil pump. The pickup isn't cracked. In reality, old engine + new oil pump would probably have been fine for a long time. Oh well. At least the crank probably isn't ruined, I might rebuild the engine so I have a nice fresh 2.5
For now, the 2.2 is going to be good though. It's one of the smoothest running suby motors I've ever met. No piston slap, valve noise, subaruy knocky grumble noise, etc. I'll put new timing belt stuff on it, and it should go for 100k miles without any problems.