I did this on my 2006 330 Express. I mounted mine close to the keel. Getting the height right is important (bottom of transducer relative to the bottom of the keel). The Manufacturer can advise you best on this. I can text you photos if you PM me your #.
As to why a transom mount? I upgraded from a very basic unit to a much better transom mount so the install was relatively easy for me. I also prefer as few holes in the bottom of my boat as possible. That said - when I do a full upgrade, I'll go with a traditional, big boat, transducer.
PM sent , appreciate the help. Reason for transom mount is due to upgrading from Airmar B60 style transducer (cheap) to more offshore style for tuna / swordfish. Also didn’t make sense to cut new holes in hull of boat and have to refill, makes more sense to install the 275 transom as you did. Looking forward to seeing pictures of how it turned out