I have a 318 that has less than 500 miles on it. I took the rockers and pushrods out of it and tried them in the 340 with the same results. I wanted to make sure it wasn't a pushrod or rocker. The same pushrod is loose. I marked all the pushrods with a sharpie and ran the motor briefly to confirm the lifters are spinning and they are all spinning about the same speed, including the one that has the loose pushrod....

Now, this is something I need to check on... Yesterday as I was doing my testing I had removed the passenger side valve cover and reinstalled it. As I was reinstalling it something lying n the intake manifold fell into the head. I immediately took the valve cover off and started looking for it. I stuck my finger into a puddle of oil in the head and felt a small disc and pulled it out. It looked almost like a watch battery. I figured it was something that fell onto the intake while I was installing the motor. I looked a little more to make sure there wasn't anything else and I found a washer. The washer was what I though fell into the head in the first place. It came off one of the bolts I used to lift the motor assembly with my cherry picker. I didn't give it much thought last night but the more I think about it I am afraid the disc was the tip of the valve. I'm going to go out in a bit and check. If that's the case, what can cause the valve tip to come off? I bough the heads used but they had little use and I and the machine shop check them over. I guess a broken valve tip would be better than a wiped cam...Not much but a little.