You put the bracket so that the point were the motor rests on the bracket is 25" from the bottom keel.
Now there is something out there about rasing the bracket a few inches for evry foot back you go??
I just put mine at 25 inches for the motor and it works fine!!
As for fixing the cut out, there will be no stress on it if the bracket is bolted thur the transom below it. So patch away!!
Just make sure that the transom it self is strong!! And you may want to add a bracket like I did to the inside of the transom, it will make sure that the brack and transom stays toghter as one!!
Like this:
you will love a full transom!! And as far as gas in the middle of the boat don't wory about it it will be fine!!