When I purchased my 1977 34' DC from Marine Trading International the salesman told me the keels were filled with marble chips and concrete. The marble chips come from a local quarry.
I wouldn't put a transducer in the keel. One of mine is located in a fairing block just under the forward bulkhead on the starboard side of the keel. That appears to be the factory position. The other is located aft just forward of the aft bulkhead on the starboard side.
I have a new one this year that will be shooting through the hull. Don't know where I'm going to put that.
34' Marine Trader