You can cobble something up to spin the driveshaft with a drill in a bucket of water (lower unit removed of course) and see just how well it pumps.
You may have a blockage in the copper tube going to the powerhead; or a blockage in the powerhead itself. Is this a salt water motor?
This PDF is for a 1970; close as I can come. It says Sea King which were made by Chrysler during those years. Probably similar.
