Are you sure? I have never removed the valley cover so I would really not know, it was just something I saw on PT.net
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ACTUALLY...you CAN replace the lifters without taking the heads off. How? Take the cam out, put a piece of cardboard or other firm media to act as a cradle in the cam journals of the block and with a magnet, pull the lifters down. They will be caught by the cardboard when they fall if they come off the magnet. You can replace them the same way you take them out.
It would be HIGHLY difficult, but it can be done. I would never recommend it.
That sounds like a disaster waiting to happen, but I see your point.