Glenn,
I really don't know about the slots in the pushrods. I would think if the old ones had them, and they appear to be factory, then it might be wise to modify the new ones.
That whole supply system to the rocker arms is pretty marginal when it comes to crud in the oil.
I have seen the rockerarm shafts themselves so plugged that you almost thought it was a solid rod. I would crack fittings in sequence as I walked toward the center oiler and verify each was leaking. When you reach a fitting that doesn't burp oil, you know the restriction in downstream.
From a technical standpoint, I don't know if new pushrods on old lifters is kosher..especially when you indicate the "foot" is a little smaller.
The sidecover area is inconvenient to work on to say the least. You don't want to have to revisit it anytime soon.
I thought I had seen mention of blasting the passages with compressed air (in reverse) in an attempt to clear them. I hope these suggestions help, maybe the pushrod guru's will be along shortly.
Stuart
