Sad to say, it is probably cam lobe going away. If you run a good oil with zinc it may well last indefinitely. Happened to me with a factory 340 and 318. Both sat for extended lengths of time. The flush will likely not do anything as long as you do regular maintenance with a good oil and filter. I like the idea of a slightly longer pushrod, but I would measure with an adjustable measuring pushrod to see what it would take to get the proper preload. Assuming hydraulic lifters and stock rockers.
