An additional thought occurs to me: Why aren't you bolting the winch
through the block? It makes very little sense to bolt a block to the deck, then screw a winch to the block. You really should be through bolting the winch, right through the bock and deck with the backing plate, etc.
Boracay has a good point about the mastic, you don't want to crush the cement by point loading, you might be better with mastic on both
the top and underside of the deck to be sure you aren't crumbling things.
What size winch are we talking about? What kind of loads are we dealing with? If it's a primary sheet winch, loads are high and you really need to do this RIGHT.