Lots of previous posts on these.
If your deck is cored you can't put it shoot-through, you have to put it in/through-hull or on the transom mount. Find out if your deck is cored.
Older transducers really don't work with the newer stuff... the newer transducers are mostly made by one company, most of the major fish finders have proprietary plug interfaces that won't work with older cables/transducers.
1976 25' O'day - "SeaWind"
Hello Sailor: "I've just learned that good boats, in good hands, are damned robust creations."