Twin Keel vs Bulb Keel
Actually, you need to be very worried about shrinkage as the lead will pull away from the steel when it shrinks and will create a cavity that is perfect to allow crevise electrolysis in the cavity between the steel and the lead. Depending on the design of the keel, this cavity can be as large as and inch or more fore and aft, and a 1/4" side to side. Beyond that the lead is free to move within this cavity giving it momentum to shift and do damage in a grounding or shift and make a lot of noise in a rolling rodestead. The more traditional way to do this is to extract the lead after the pour, and to fill the cavity with a semi-soft material like hot tar or concrte (I don''t especially like concrete as it traps moistre against the steel) and then put the lead back in. This fill helps to restrain the lead as well as eliminate the cavity. Frankly it would be easier to extract the lead and not even bother with keeping the steel which will be a maintenance pain in the butt over the life of the boat.