They're 3/8 studs as-equipped from AFR. I originally set to 0-lash then 1/2 turn. Then went back in after the engine was hot and re-set while running. Bottom end is 100% stock. I am confident its not a rod bearing going out - its a definite "tick" versus a knock. Then again, with this engine anything is possible.
I pulled all the lifters last night and checked them, as well as all the cam journals. The cam looks perfect. 15 of 16 lifters look "new", but one looked like this:
The grooves in the roller are deep enough to where you can feel them with your fingernail. These lifters were run for 200-300 miles. Again, the corresponding cam journal looks perfect. Zero grooves or damage.
I have purchased an entire set of new lifters and will be installing them today.