I'm sure a P&G Valve gapper would work on a HiPo if it was configured correctly. The problem is that those tools have a dozen or so different bases, 3 inner barrels, at least 2 outer barrels, and 3 different springs. They could be configured to work on a ton of different engines from 4 cylinders to Cummins diesels at finding out what combination of parts you need for a small block ford nowadays could be trying to say the least. When they were new, you looked up your application in a P&G catalog and it told you what combination of parts you needed.
When configured correctly, they are a cool little tool, and you can set your actual lash precisely as the tool measures the actual lash on a hot, idling engine.
I'll try and dig some out and post some pics, John