FWIW,
I have 1.6 aluminum Proform rockers on my 406 w/vortec heads. It's a solid roller and I have beat it up pretty hard, no failures in the last 9+ years it's been running.
I believe mine were 3/8. I have taller valve covers on them. I do believe they hit the valve cover at the corners (front & rear) so I had to grind them down a little where their square edge is. I suppose this can change based on the exact valve cover used.. Mine were some billet aluminum ones.
-Dale
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