Had same issue on the release on my 70. The thread on that one was/is normal right hand. The kick in the ding ding was that it is a nylon screw
with an O-ring on the inside AND it had nothing left for threads on it.. Because I had it all apart to replace the lower swivel mount It was in pieces and easy to get at. I had to grab under the screw head with vise grips, squeeze just hard enough to keep from crushing the nylon and pull out (that's the key) and turn with a 10mm socket. The boogerd threads had no yank left in them and the O ring held the screw in place with tenacity that was impressive.
If you don't want to pull her all to pieces Chris, you're gonna have to fab up a tool to get in there under the screw head to pry out as you turn. Unless it is a different trim screw then mine, the engine mount bracket is going to have to come off anyway because the head, 10mm socket size, will not fit through the hole in the bracket..
Nothing wrong with over sizing the hole but that'd be a big drill and a mess if you missed..
Now, I just woke up (gotta work tonight) and you could have something COMPLETELY different. Having doubts about the 10mm size too.... A quick look at the parts list online will show you what you are fighting against.. Mercurypartsonline has good schematics and parts breakdowns..
Good luck....
