You may have some left SI blockage with the drain plug and the livewell screen...
You might want to test with double sticky tape before drilling holes...(just be sure it's stuck good)...
Some have modified their drain plug and/or livewell screens to eliminate blockage problems...
Some have installed a backer board as a spacer between the transom and the HDSI....
You also may need to think about the AS SIDB Y cable setup with a shoot thru 2d xducer so the HDSI wouldn't have to be mounted below the boat hull where it could possibly be damaged by waterborn debri...
I would suggest your idea of mounting first...and then deal with issues "if" they arise...
Rickie