Those are slotted hex nuts, similar to a lock nut but not really a locking nut. They were used in vibration areas like A/C mounts, water pump mounting, P/S mounts, any place a nut could have vibrated loose. The slotted head bolts you see, called PLACE-BOLTS,
https://www.aftfasteners.com/place-bolts-specifications-dimensions/ are pretty neat technology, the nuts may do the same thing. The slots likely act like teeth gripping just enough to not back off.
Pontiac built all their engines with the same rod bolts and nuts, 3/8-24 tall nut, non-locking, straight nut. From inline 6 and 8 and every size V8 made, all used the same nuts torqued to 45 ft lbs. My guess is the design of the bolt is what keeps the nuts tight, not the nut.
If you decide to change them, use a tall nut, or even better yet a new bolt and nut set, but you will need to have the rod resized if you go swapping bolts. You could get away with new nuts, but you need to check each rod carefully when assembling the engine. The bolts locate the caps, changing them will distort them. There is a video's on youtube where a guy changes rod bolts on a Ford engine without resizing the hole and shows how bad they go out of round.
