The 3rd generation GM six.

[shp4man]

My truck had a 250 CID engine when I acquired it. Turns out it was an old rebuilt engine by a local company (Jasper). Figured that out when I took the valve cover off for a reseal. Clean as a whistle inside.
It had some ignition and carburetor issues, as you would expect, but those have been addressed.
Anyway, it keeps up with modern traffic pretty well, uses no oil, and is completely reliable. I'd drive it across the country, no problem.
Alls I'm sayin' is don't trash the old six unless you're a drag racer or something. It's a damn good engine.