I believe I've found a decent source for a transducer for the '79. It turns out I need the 2 wire transducer because even though I have a 3 wire now I do not have the resume valve which is a separate piece that bolts to the transducer. The guy I am getting it from rebuilds them and had a lot of good information when I talked to him. My mistake was assuming the equipment in my truck actually came in the truck originally which led my search awry. Check out this guy's store: https://www.ebay.com/str/celiafromthenutcompany86 I should have the new transducer on Saturday and I'll report the results as soon as I can.