[Barry_R]

Lykins methodology for measurement is sound. Most FE rockers that have ball adjusters will use a 3/8 cup on the top of the pushrod. Many lifters can actually accommodate multiple ball diameters, but its commonly assumed that you'll want a 5/16 ball at the bottom.

The 8.350-8.400 bottom of cup measurements are pretty normal for hydraulic roller FE engines - so you are not off in that regard.

None of this will matter all that much because the Pro-Comp rockers will probably be the weakest link here. I've had a customer provided set or two here and I refused to use them on the engine. Really poor machining covered up with pretty plating. Caveat emptor and all that...I know they are at a low price. I truly don't intend to rain on that parade, but you should at least have your umbrella ready....