So much for the " it came out of a running, driving car. Drop it in and go."

Don't feel bad, did the same thing.
I bought a "running" slant from a friend of a friend.
I got it home and started to clean it up. Before to long it was "I should reseal that, and replace this"
Long story short, I ended up completely rebuilding the engine.
The hell of it is, I wasn't going to pull the crank, but that rear main seal was being stubborn, and I messed it up so I had to replace it.
Glad I did, the rings were like glass. They shattered when I pulled the pistons.
If I hadn't, that thing would smoked like a train, if it would have even started.