Back in the mid-60s, I had, first, a '64 Custom/427 and then a 65-1/2 Mustang with the 'K' engine. To keep up on maintenance, I bought a P&G valve-gapping tool. I used it on the 427, and then, after trading the '64 to Brother Lon, I got a new 'barrel' to fit the 'K' 271-289 engine.
Although I made the effort to have the engine at operating temp before checking valve lash, and always went through the same set of conditions, I always found one or more valves that needed a bit of adjustment. Since the car that preceded the '64 was a '54 Customline into which I'd swapped a 312 with both the factory race dual-quads and a Paxton VS 59 blower (also the race set-up), I was used to having to 'run the valves' every once in a while---the Y-block series came only with 'nail-head' solid lifters. And I'm sure that many solid lifter cars from the time went years between adjustments. But I could always hear that one-or-two valves with a louder tick than the rest. I actually had to replace adjuster screws on the 427, and replace rockers when the oversize screws would lose their interference fit. In fact, due to access to the tool-room at T&C Livonia, we ultimately made lock nuts tor the FE adjusting screws, and spot-faced the tops of the rockers to provide a flat for the nut to lock against.
But then, we also made High-volume oil pumps too, before you could get one from Melling. (The essence of hot-rodding is improvisation!)
KS