Some of the roller blocked truck/van 5.0's that had flat tappet cams didn't have the lifter valley retainer bolt bosses drilled and tapped, some even didn't have the bosses fully cast, I have one of these, the bosses are there, but not fully formed. The last two blocks like this I've aquired, one had the bosses drilled and tapped (but plugged with carbon gunk), the other had the "half-cast" bosses. One was an 88 block, the other an 89. Ditto on the knock sensor hole too, mentioned by 60oneton, although I think you can drill and tap this on any of the blocks.