I know this thread is a little old and you all may have ( I hope) solved
your problems by now. However, reading this made me remember something I had come across recently in an original brochure (online) for the Ingrid, describing the construction of the fibreglass hulls by Bluewater in the 1970's and early 80's. Here's the quote:
BALLAST: 8,000 pounds total. Approximately 7300 pounds steel
and 700 pounds resin and sand. Placed inside the keel
full fore and aft from bobstay iron to the stern shoe, approximately 20" deep at the deepest part. Glassed over with 1 layer 2 oz. Mat, 6 layers roving to provide a double-bottomed boat, box beam rigidity, immense strength. Cooper screen
laid under the ballast with copper cables
up to each bulkhead for a ground plane for long distance radio. Also serves as grounding for all fittings.
Good luck in finding the cables