Mounting it on the step down would not prevent a stump from taking the xducer out if the boat drifts a bit sideways, or any angle where the lower unit doesn't catch the stump first due to a gust of wind or wake, etc.
I mounted a 2d xducer on a tin hull with step transom using a "stern saver" (google it). It worked on plane and at idle, but I did not mount it directly behind/under the LU but in the normal position away from the LU.
Might consider a semi permanent/temp mount that would bend/flex before doing any serious damage that you deploy when you get to your fishing hole
GO BIG ORANGE !
I meant to behave, but there were just way too many other options available at the time.