The tappets will not come out from the tappet cover. You can only see teh top of the tappets above the tappet bores. The 2 main ways of changing them is with the windowed pipe, baling wire, and a small magnet to fit into the tappets. Although this method can be very frustrating at times, and if you drop one inside the engine, the oil pan will have to come off to retreive it. Although if you are by yourself, this will probably be the easiest method.
Another way would be to raise the engine from its motor mounts and remove the oil pan. To do this with the block adapter in place, the pan will drop down just low enough to clear the block adapter once you have the engine raised out of its motor mounts. In some cases though you will have to remove the oil pump pick up tube in order to pull the pan completely out. Once the pan is removed you can drop the engine back down into its motor mounts for saftey reasons (thats just me though because of bad luck with heavy things falling). This will allow you to directly place the tappets into the engine yourself from underneith the truck, but you will need someone up top to place the wood dowl pins, hose clamps, zip ties, or what ever method you use to hold the tappets in place.
