After your deck is done, the bolt should be torqued enough so that the clamping force will not allow the clutch to move. Bolts are not meant to be used in a shear situation. They are meant to clamp (why a bolt should never be used as a clevis pin). If you need a block to keep the clutch from moving it's because your bolts aren't tight enough or your deck isn't strong enough to not deflect. I have no doubt your deck will be strong. Your bolts should hold if they are torqued properly.
I look at potting through deck holes as not only waterproofing the core but also providing an extremely solid bolting core. Something that will not compress. I use silica as my filler for these holes to provide the strongest bolting core. I am not shy about hogging out the core to be quite oversized. I've even considered installing stainless thick-wall tubes into the deck to give a clamping core. Haven't gone that far yet.
my $.02
