Everybody seems to have a different take on how to winterize the block but it sounds to me like you've gone the traditional way which is how I do it also. You state that you've drained the block and the exhaust. By this I'm assuming that you've disconnected the hose that runs between the two exhaust manifolds and let it drain and drained the block on both sides of the engine underneath the exhausts right? I ask because not everyone knows there are two drains for the block and draining both is critical.
Assuming you have completed both these steps correctly the next step would be to locate the large hoses just under the thermostat. One will go to the raw water pump the other to the block. You want to the one going to the block. Take the hose off at the top and start pouring marine antifreeze (I use the -60 antifreeze with corrosion protection that West Marine sells) into the hose until it start coming out the housing where you disconnected the hose from. When you see the antifreeze coming out the housing you'll know that the antifreeze has filled the block all the way to the level of the housing. Be sure to also disconnect the hose coming from the bottom of the boat to the trans cooler and clean out any debris that may be there. This will also ensure there isn't any water left in the trans cooler. I also like to pull the impeller for inspection/replacement. This will also ensure there isn't any water left in the pump to freeze. I will typically also remove the hoses going to the exhaust manifolds and pour antifreeze into to them also. This is kind of overkill as the design of the manifolds should allow for complete drainage when you disconnect the drain hose. For me though it's "better safe than sorry".
This is the procedure I've used for years on Correct Crafts, Malibus, MasterCrafts and Supras that I've worked on for myself and friends without a single incident. It's pretty straight forward and you know the block is filled when your done. I will say though that I have not worked on an Ilmor however I doubt it would be much different.