Hello,
Most the time it never happens, but have you checked the pushrod length on the cylinder (s)? I learned that the FE made a change in the length of it's pushrods in about 1968. The earlier ones are .060" longer.. Also the valves are set at a certain height, by the machinist,for the pushrod length to be correct on installation.
I would compress the lifters on that cylinder, and check the rocker to valve tip. It should be between .100" and .200". This will center the lifter. To make sure that you are correct in adjusting them properly. Turn the engine over till the cylinder you want to check, Exhaust valve just starts to open, check the Intake. Then turn the engine over till the Intake is fully open, and then just starts to close.. Check the Exhaust.. If they are more than .200" the pushrod is short.. If it's under .100" the pushrod is to long.
May not help, but it's good info, Ray
Anybody else out there do it this way? or any other way that's better?!