I used the same parts as were used by the builder when the original owner had the engine was rebuilt in 1998. These:
https://www.summitracing.com/parts/cca-928-16 (and yes, they are super stiff, but I assume what the custom cam is spec'd for).
Tonight, I took the valve cover off to check the valve train, and discovered the stud closest to the fourth cylinder had worked itself loose. Turns out, it is completely stripped.
Not sure how I was able to torque it to spec when putting things back together - it must have been wedged well enough and then wiggled loose once the engine was running. I want to believe this is cause of the sounds. I've got a Time-Sert ordered to repair the threads, so I'll find out hopefully this weekend. Valve train itself still appears nice and straight.