That big score mark in line with the oil passage tells the tale. A pretty large piece of debris from somewhere else in the engine found its way to the rod bearing, and created a divot that spun around between the crank and the bearing. This created more debris that continued to damage the bearing and the crankshaft. That's too nice a truck to butcher up with a hotrod job- - - - - -either find a serviceable 6-cylinder to drop in temporarily while you do a buildup on the current engine, or see if you can find a 261 engine somewhere for a modest upgrade while staying with a fairly original appearance. I hate to see good-condition original-appearing trucks hacked up with a V-8 IFS "improvement"!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!