Just had one put in my 263 Chase, which is the same hull as a 273 Chase and maybe the same as yours?
I already had a transducer (that I wanted to keep) and a livewell pickup on each side of the keel (about 1 to 1.5 feet off the keel), so they mounted the new one further out on the port side (maybe 2.5 to 3 feet off the keel). It held bottom only to about 15 knots, and I wasn't happy. I had them move the livewell pickup two feet further aft, and put the new ducer where it was (plus repair the new hole they put in the boat :bang ). Now it holds bottom solidly to 40 knots!
So, keep it close to the keel! You can't put it REAL close to the keel because the hull "rounds out" at the bottom, and you need a flat surface for the flush-mount ducer.
Good luck!