I wouldn't spend the money on the bench tester. 3-4K miles seems reasonable that the springs would lose a little on the seat. The tester you have is fine and for your reference only. If you test them all with the same tester and get said results, that is your reference point for future checking. Shimming the spring cups is common. With a used head, installed height is probably not going to be perfect and you should concentrate more on not binding the spring. The air method works great as does the rope. I like the air better because the valves pop back up when you smack 'em a little. When you apply the air you can hold the engine with a wrench on the balancer and feel which way it is trying to turn. You can leave your ratchet or breaker bar on the engine and rest it on whichever frame rail will stop the engine from turning. It's a pretty common practice to run mech. cams on the outer spring only and put the inners on it after break in. I've never heard of anyone letting a valve drop in the cyl. As said before, you can do two cyls. at a time. If you really want everything perfect, buy the new head gaskets and do them on the bench after your machinist sets all the valve heights again. Sorry about the lifter prob. That sucks!