How I found my worn guides was first with a auto stethascope locating the area of the noise. This put me right on the number four and 5 intake valves then by removing the spark plugs and rotating the engine by hand and watching the movement of the top of the valve as it went through it's lift cycle. When I removed the head off the truck and disassembled the spring off those valves it was very clear what the issue was. As soon as the valve came off the seat, the top of the valve could be wiggled 3/16" of an inch. Oddly enough the seats were still fine. The engine idled with 21" vacuum and had good compression too.

1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
In the DITY Gallery
Video of the 261 running

1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration