My '74 CC is original and the batt is mounted under console-you should be able to find a couple of screw holes where the strap securing the battery box was screwed down if your's is like mine. The thru-hulls you mention are on mine-the two on the transom are the actual drains for 1) fuel over-flow from neck of fuel-fill and 2)drain for any water accumulation in depressed area around fuel fill. they are two totaly indepedent but identical drains--and there should also be 2 thru hulls on the side which are obviously "teed" into these drain lines which serve as a "breather" ( just like any house drain is plumbed)-regards
larry