I enjoyed this thread and its happy outcome because I have "been there/done that". About 35 years ago (I was 17) I did my first cam swap on my F150/ 400cid. I was careful as a mouse but when I got done I couldn't get it timed or running to save my butt. Just prior to tearing back into it I realized I was using the wrong mark. You never saw a happier young man than when it lit, and went bumpety-bumpety-bump.

And yes, balancer rings DO slip occasionally. I encountered one on a 350 Chevy about 25 years ago. It was a real head scratcher until I figured it out.

 

It's when you've been there done that but are presently having a brain fart that you feel a little less smart.

 

You gotta have a brain fart once in a while, otherwise your head will explode!