Thank you so much. These are definitely from my father's side. Luckily both of my parents have large families who seemed to have jumped on the DNA bandwagon, so it's somewhat easy to isolate which branch most close relations are.

My father's father's branch doesn't seem helpful though. The man is now deceased, so I know it might mar the memory of "dear Grandpa". He looks to have been married with 4 kids. I know that adds to the sensitivity, considering that he presumably was having an affair. I figured with so much time having passed, someone might be curious and less sensitive.

Gedmatch has one big group being managed by the same woman. She responded a year ago that she'd "look into it" and now...nothing. I've emailed again...nothing.

I found a woman on Ancestry who is a close match (356.5‎ cM shared). She said her tree was on myheritage. Fine, I make an account on myheritage to peek at her tree. I emailed her stating the possible connection. No response. I don't want to scare people off.