I had a small block (basically stock)built by a shop to install into my street rod. After about 1000 miles, the motor has developed a loud lifter knock upon start up and at speed after driving about 10 minutes.

When the motor was installed I had a single electric fan mounted to an aftermarket aluminum radiator. The single fan would not cool the motor in traffic so I added a second, both pullers. These fans were mounted to a sheetmetal shroud loosely based on a Be Cool setup I had seen on a blown big block car. The setup cooled the car as expected.

THe engine builder is convinced the fans and shroud are cooking the motor and wants a conventional motor driven fan and standard shroud installed before he commits to replacing the lifters under a very generous warranty.

What are your experiences with stock vs. aftermarket fan/shroud setups?

If I had paint peeling on the hood, it wouldn't have anything to do with my worn out tires.
The above makes just as much sense as your situation. Even a overheating engine will not make a lifter or cam go bad, That is an oiling or "cam break in" issue.
Good luck with the fan shroud though.

oil pressure is constantly between 70 and 75 psi.

When the issue was first discussed with the builder, I was told the lifter noise was possibly a result of an oversized lifter bore allowing excessive movement.

It's certainly not a lack of PSI . I would see what you have for preload on em & press down on each of the rocker arms (the pushrod side) w a wooden hammer handle while it's idling & see if you can isolate the bad one (or ones) (it'll get quieter). One or more of the lifters may have went bad but I'd check preload 1st