Always set them with the engine running but worn rockers and valve stems will certainly chew up a feeler gauge. A well tuned stovebolt 6 will easily idle at 450rpm (or slower) so if you first loosen the set nuts just a bit you can pick up on getting a screwdriver into the moving adjuster. They make a tool for doing this; it has a 5/8 12 point box wrench end for the jam nut and a spring loaded screw driver with a round knurled knob to turn the adjuster screw. You place the box end on the nut and then rotate the knob till the blade drops in the slot and then screw the adjuster in or out.

Evan