340 Rod questions

The oil rings' function is obvious. The 2nd ring is as much for oil control as anything. So, yes, you could put in just the top ring to run around.... I have seen and heard of lots worse LOL. But you can only buy the rings as sets.

The comment on the ridge was if you put in a 'thicker' rod bearing, it will push the rings up a tad higher in the bore.... I am sure that folks have gotten away with it, but it also has been known to break a top ring. You just don't want to break the new top ring and score the cylinder before you get it back apart. Sounds like experiment time! Be brave!