[tubman]

If the car really "runs nice", I'd replace the seal and leave the rest alone. I might not even replace the seal. You don't know what you might run into if you decide to open it up. If there's a problem (low compression, low oil pressure, coolant loss, or something like that), then go ahead. I would be afraid of a crack in the block that is currently not leaking being discovered. You really can't put it back together again, and rebuilding it with a known cracked block is not recommended. And then what if you can't find another block?